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<div class="headertitle"><div class="title">modular_inverse_fermat_little_theorem.cpp File Reference</div></div>
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</div><!--header-->
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<div class="contents">
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<p>C++ Program to find the modular inverse using <a href="https://en.wikipedia.org/wiki/Fermat%27s_little_theorem" target="_blank">Fermat's Little Theorem</a>
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<a href="#details">More...</a></p>
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<div class="textblock"><code>#include <cassert></code><br />
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<code>#include <cstdint></code><br />
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<code>#include <iostream></code><br />
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</div><div class="textblock"><div class="dynheader">
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Include dependency graph for modular_inverse_fermat_little_theorem.cpp:</div>
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<div class="dyncontent">
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<div class="center"><iframe scrolling="no" loading="lazy" frameborder="0" src="../../d3/d1e/modular__inverse__fermat__little__theorem_8cpp__incl.svg" width="255" height="126"><p><b>This browser is not able to show SVG: try Firefox, Chrome, Safari, or Opera instead.</b></p></iframe></div>
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<p><a href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html">Go to the source code of this file.</a></p>
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<table class="memberdecls">
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<tr class="heading"><td colspan="2"><h2 id="header-namespaces" class="groupheader"><a id="namespaces" name="namespaces"></a>
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Namespaces</h2></td></tr>
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<tr class="memitem:math" id="r_math"><td class="memItemLeft" align="right" valign="top">namespace  </td><td class="memItemRight" valign="bottom"><a class="el" href="../../dd/d47/namespacemath.html">math</a></td></tr>
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<tr class="memdesc:dd/d47/namespacemath"><td class="mdescLeft"> </td><td class="mdescRight">for assert <br /></td></tr>
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<tr class="memitem:modular_5Finverse_5Ffermat" id="r_modular_5Finverse_5Ffermat"><td class="memItemLeft" align="right" valign="top">namespace  </td><td class="memItemRight" valign="bottom"><a class="el" href="../../d7/d50/namespacemodular__inverse__fermat.html">modular_inverse_fermat</a></td></tr>
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<tr class="memdesc:d7/d50/namespacemodular__inverse__fermat"><td class="mdescLeft"> </td><td class="mdescRight">Calculate modular inverse using Fermat's Little Theorem. <br /></td></tr>
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</table><table class="memberdecls">
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<tr class="heading"><td colspan="2"><h2 id="header-func-members" class="groupheader"><a id="func-members" name="func-members"></a>
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Functions</h2></td></tr>
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<tr class="memitem:a1c62a87e79f29b98d8fe5b12eacdd928" id="r_a1c62a87e79f29b98d8fe5b12eacdd928"><td class="memItemLeft" align="right" valign="top">std::int64_t </td><td class="memItemRight" valign="bottom"><a class="el" href="#a1c62a87e79f29b98d8fe5b12eacdd928">math::modular_inverse_fermat::binExpo</a> (std::int64_t a, std::int64_t b, std::int64_t m)</td></tr>
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<tr class="memdesc:a1c62a87e79f29b98d8fe5b12eacdd928"><td class="mdescLeft"> </td><td class="mdescRight">Calculate exponent with modulo using binary exponentiation in \(O(\log b)\) time. <br /></td></tr>
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<tr class="memitem:aba4929409fee35c3cb559e962a544b3e" id="r_aba4929409fee35c3cb559e962a544b3e"><td class="memItemLeft" align="right" valign="top">bool </td><td class="memItemRight" valign="bottom"><a class="el" href="#aba4929409fee35c3cb559e962a544b3e">math::modular_inverse_fermat::isPrime</a> (std::int64_t m)</td></tr>
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<tr class="memdesc:aba4929409fee35c3cb559e962a544b3e"><td class="mdescLeft"> </td><td class="mdescRight">Check if an integer is a prime number in \(O(\sqrt{m})\) time. <br /></td></tr>
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<tr class="memitem:ae7e807e02c65c6fffd6162b4c66290c2" id="r_ae7e807e02c65c6fffd6162b4c66290c2"><td class="memItemLeft" align="right" valign="top">std::int64_t </td><td class="memItemRight" valign="bottom"><a class="el" href="#ae7e807e02c65c6fffd6162b4c66290c2">math::modular_inverse_fermat::modular_inverse</a> (std::int64_t a, std::int64_t m)</td></tr>
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<tr class="memdesc:ae7e807e02c65c6fffd6162b4c66290c2"><td class="mdescLeft"> </td><td class="mdescRight">calculates the modular inverse. <br /></td></tr>
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<tr class="memitem:aa8dca7b867074164d5f45b0f3851269d" id="r_aa8dca7b867074164d5f45b0f3851269d"><td class="memItemLeft" align="right" valign="top">static void </td><td class="memItemRight" valign="bottom"><a class="el" href="#aa8dca7b867074164d5f45b0f3851269d">test</a> ()</td></tr>
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<tr class="memdesc:aa8dca7b867074164d5f45b0f3851269d"><td class="mdescLeft"> </td><td class="mdescRight">Self-test implementation. <br /></td></tr>
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<tr class="memitem:ae66f6b31b5ad750f1fe042a706a4e3d4" id="r_ae66f6b31b5ad750f1fe042a706a4e3d4"><td class="memItemLeft" align="right" valign="top">int </td><td class="memItemRight" valign="bottom"><a class="el" href="#ae66f6b31b5ad750f1fe042a706a4e3d4">main</a> ()</td></tr>
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<tr class="memdesc:ae66f6b31b5ad750f1fe042a706a4e3d4"><td class="mdescLeft"> </td><td class="mdescRight">Main function. <br /></td></tr>
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</table>
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<a name="details" id="details"></a><h2 id="header-details" class="groupheader">Detailed Description</h2>
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<div class="textblock"><p>C++ Program to find the modular inverse using <a href="https://en.wikipedia.org/wiki/Fermat%27s_little_theorem" target="_blank">Fermat's Little Theorem</a> </p>
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<p>Fermat's Little Theorem state that </p><p class="formulaDsp">
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\[ϕ(m) = m-1\]
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</p>
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<p> where \(m\) is a prime number. </p><p class="formulaDsp">
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\begin{eqnarray*} a \cdot x &≡& 1 \;\text{mod}\; m\\
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x &≡& a^{-1} \;\text{mod}\; m
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\end{eqnarray*}
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</p>
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<p> Using Euler's theorem we can modify the equation. </p><p class="formulaDsp">
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\[ a^{ϕ(m)} ≡ 1 \;\text{mod}\; m
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\]
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</p>
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<p> (Where '^' denotes the exponent operator)</p>
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<p>Here 'ϕ' is Euler's Totient Function. For modular inverse existence 'a' and 'm' must be relatively primes numbers. To apply Fermat's Little Theorem is necessary that 'm' must be a prime number. Generally in many competitive programming competitions 'm' is either 1000000007 (1e9+7) or 998244353.</p>
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<p>We considered m as large prime (1e9+7). \(a^{ϕ(m)} ≡ 1 \;\text{mod}\; m\) (Using Euler's Theorem) \(ϕ(m) = m-1\) using Fermat's Little Theorem. \(a^{m-1} ≡ 1 \;\text{mod}\; m\) Now multiplying both side by \(a^{-1}\). </p><p class="formulaDsp">
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\begin{eqnarray*} a^{m-1} \cdot a^{-1} &≡& a^{-1} \;\text{mod}\; m\\
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a^{m-2} &≡& a^{-1} \;\text{mod}\; m
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\end{eqnarray*}
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</p>
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<p>We will find the exponent using binary exponentiation such that the algorithm works in \(O(\log n)\) time.</p>
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<p>Examples: -</p><ul>
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<li>a = 3 and m = 7</li>
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<li>\(a^{-1} \;\text{mod}\; m\) is equivalent to \(a^{m-2} \;\text{mod}\; m\)</li>
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<li>\(3^5 \;\text{mod}\; 7 = 243 \;\text{mod}\; 7 = 5\) <br />
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Hence, \(3^{-1} \;\text{mod}\; 7 = 5\) or \(3 \times 5 \;\text{mod}\; 7 = 1 \;\text{mod}\; 7\) (as \(a\times a^{-1} = 1\)) </li>
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</ul>
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<p class="definition">Definition in file <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html">modular_inverse_fermat_little_theorem.cpp</a>.</p>
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</div><a name="doc-func-members" id="doc-func-members"></a><h2 id="header-doc-func-members" class="groupheader">Function Documentation</h2>
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<a id="a1c62a87e79f29b98d8fe5b12eacdd928" name="a1c62a87e79f29b98d8fe5b12eacdd928"></a>
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<h2 class="memtitle"><span class="permalink"><a href="#a1c62a87e79f29b98d8fe5b12eacdd928">◆ </a></span>binExpo()</h2>
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<div class="memitem">
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<td class="memname">std::int64_t math::modular_inverse_fermat::binExpo </td>
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<td>(</td>
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<td class="paramtype">std::int64_t</td> <td class="paramname"><span class="paramname"><em>a</em></span>, </td>
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</tr>
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<tr>
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<td class="paramkey"></td>
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<td></td>
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<td class="paramtype">std::int64_t</td> <td class="paramname"><span class="paramname"><em>b</em></span>, </td>
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<td class="paramkey"></td>
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<td></td>
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<td class="paramtype">std::int64_t</td> <td class="paramname"><span class="paramname"><em>m</em></span> )</td>
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</div><div class="memdoc">
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<p>Calculate exponent with modulo using binary exponentiation in \(O(\log b)\) time. </p>
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<dl class="params"><dt>Parameters</dt><dd>
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<table class="params">
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<tr><td class="paramname">a</td><td>The base </td></tr>
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<tr><td class="paramname">b</td><td>The exponent </td></tr>
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<tr><td class="paramname">m</td><td>The modulo </td></tr>
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</table>
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</dd>
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</dl>
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<dl class="section return"><dt>Returns</dt><dd>The result of \(a^{b} % m\) </dd></dl>
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<p class="definition">Definition at line <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html#l00067">67</a> of file <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html">modular_inverse_fermat_little_theorem.cpp</a>.</p>
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<div class="fragment"><div class="line"><span class="lineno"> 67</span> {</div>
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<div class="line"><span class="lineno"> 68</span> a %= m;</div>
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<div class="line"><span class="lineno"> 69</span> std::int64_t res = 1;</div>
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<div class="line"><span class="lineno"> 70</span> <span class="keywordflow">while</span> (b > 0) {</div>
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<div class="line"><span class="lineno"> 71</span> <span class="keywordflow">if</span> (b % 2 != 0) {</div>
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<div class="line"><span class="lineno"> 72</span> res = res * a % m;</div>
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<div class="line"><span class="lineno"> 73</span> }</div>
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<div class="line"><span class="lineno"> 74</span> a = a * a % m;</div>
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<div class="line"><span class="lineno"> 75</span> <span class="comment">// Dividing b by 2 is similar to right shift by 1 bit</span></div>
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<div class="line"><span class="lineno"> 76</span> b >>= 1;</div>
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<div class="line"><span class="lineno"> 77</span> }</div>
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<div class="line"><span class="lineno"> 78</span> <span class="keywordflow">return</span> res;</div>
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<div class="line"><span class="lineno"> 79</span>}</div>
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</div><!-- fragment -->
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</div>
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</div>
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<a id="aba4929409fee35c3cb559e962a544b3e" name="aba4929409fee35c3cb559e962a544b3e"></a>
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<h2 class="memtitle"><span class="permalink"><a href="#aba4929409fee35c3cb559e962a544b3e">◆ </a></span>isPrime()</h2>
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<div class="memitem">
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<td class="memname">bool math::modular_inverse_fermat::isPrime </td>
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<td>(</td>
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<td class="paramtype">std::int64_t</td> <td class="paramname"><span class="paramname"><em>m</em></span></td><td>)</td>
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<td></td>
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</div><div class="memdoc">
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<p>Check if an integer is a prime number in \(O(\sqrt{m})\) time. </p>
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<dl class="params"><dt>Parameters</dt><dd>
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<table class="params">
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<tr><td class="paramname">m</td><td>An intger to check for primality </td></tr>
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</table>
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</dd>
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</dl>
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<dl class="section return"><dt>Returns</dt><dd>true if the number is prime </dd>
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<dd>
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false if the number is not prime </dd></dl>
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<p class="definition">Definition at line <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html#l00086">86</a> of file <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html">modular_inverse_fermat_little_theorem.cpp</a>.</p>
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<div class="fragment"><div class="line"><span class="lineno"> 86</span> {</div>
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<div class="line"><span class="lineno"> 87</span> <span class="keywordflow">if</span> (m <= 1) {</div>
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<div class="line"><span class="lineno"> 88</span> <span class="keywordflow">return</span> <span class="keyword">false</span>;</div>
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<div class="line"><span class="lineno"> 89</span> } </div>
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<div class="line"><span class="lineno"> 90</span> <span class="keywordflow">for</span> (std::int64_t i = 2; i * i <= m; i++) {</div>
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<div class="line"><span class="lineno"> 91</span> <span class="keywordflow">if</span> (m % i == 0) {</div>
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<div class="line"><span class="lineno"> 92</span> <span class="keywordflow">return</span> <span class="keyword">false</span>;</div>
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<div class="line"><span class="lineno"> 93</span> }</div>
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<div class="line"><span class="lineno"> 94</span> }</div>
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<div class="line"><span class="lineno"> 95</span> <span class="keywordflow">return</span> <span class="keyword">true</span>;</div>
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<div class="line"><span class="lineno"> 96</span>}</div>
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</div><!-- fragment -->
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</div>
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</div>
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<a id="ae66f6b31b5ad750f1fe042a706a4e3d4" name="ae66f6b31b5ad750f1fe042a706a4e3d4"></a>
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<h2 class="memtitle"><span class="permalink"><a href="#ae66f6b31b5ad750f1fe042a706a4e3d4">◆ </a></span>main()</h2>
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<div class="memitem">
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<div class="memproto">
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<tr>
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<td class="memname">int main </td>
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<td>(</td>
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<td class="paramtype">void</td> <td class="paramname"><span class="paramname"><em></em></span></td><td>)</td>
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<p>Main function. </p>
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<dl class="section return"><dt>Returns</dt><dd>0 on exit </dd></dl>
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<p class="definition">Definition at line <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html#l00137">137</a> of file <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html">modular_inverse_fermat_little_theorem.cpp</a>.</p>
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<div class="fragment"><div class="line"><span class="lineno"> 137</span> {</div>
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<div class="line"><span class="lineno"> 138</span> <a class="code hl_function" href="#aa8dca7b867074164d5f45b0f3851269d">test</a>(); <span class="comment">// run self-test implementation</span></div>
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<div class="line"><span class="lineno"> 139</span> <span class="keywordflow">return</span> 0;</div>
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<div class="line"><span class="lineno"> 140</span>}</div>
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<div class="ttc" id="amodular__inverse__fermat__little__theorem_8cpp_html_aa8dca7b867074164d5f45b0f3851269d"><div class="ttname"><a href="#aa8dca7b867074164d5f45b0f3851269d">test</a></div><div class="ttdeci">static void test()</div><div class="ttdoc">Self-test implementation.</div><div class="ttdef"><b>Definition</b> <a href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html#l00122">modular_inverse_fermat_little_theorem.cpp:122</a></div></div>
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<h2 class="memtitle"><span class="permalink"><a href="#ae7e807e02c65c6fffd6162b4c66290c2">◆ </a></span>modular_inverse()</h2>
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<td class="memname">std::int64_t math::modular_inverse_fermat::modular_inverse </td>
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<td>(</td>
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<td class="paramtype">std::int64_t</td> <td class="paramname"><span class="paramname"><em>a</em></span>, </td>
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<td class="paramtype">std::int64_t</td> <td class="paramname"><span class="paramname"><em>m</em></span> )</td>
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<p>calculates the modular inverse. </p>
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<dl class="params"><dt>Parameters</dt><dd>
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<tr><td class="paramname">a</td><td>Integer value for the base </td></tr>
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<tr><td class="paramname">m</td><td>Integer value for modulo </td></tr>
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<dl class="section return"><dt>Returns</dt><dd>The result that is the modular inverse of a modulo m </dd></dl>
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<p class="definition">Definition at line <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html#l00103">103</a> of file <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html">modular_inverse_fermat_little_theorem.cpp</a>.</p>
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<div class="fragment"><div class="line"><span class="lineno"> 103</span> {</div>
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<div class="line"><span class="lineno"> 104</span> <span class="keywordflow">while</span> (a < 0) {</div>
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<div class="line"><span class="lineno"> 105</span> a += m;</div>
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<div class="line"><span class="lineno"> 106</span> }</div>
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<div class="line"><span class="lineno"> 107</span> </div>
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<div class="line"><span class="lineno"> 108</span> <span class="comment">// Check for invalid cases</span></div>
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<div class="line"><span class="lineno"> 109</span> <span class="keywordflow">if</span> (!isPrime(m) || a == 0) {</div>
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<div class="line"><span class="lineno"> 110</span> <span class="keywordflow">return</span> -1; <span class="comment">// Invalid input</span></div>
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<div class="line"><span class="lineno"> 111</span> }</div>
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<div class="line"><span class="lineno"> 112</span> </div>
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<div class="line"><span class="lineno"> 113</span> <span class="keywordflow">return</span> <a class="code hl_function" href="../../de/dcf/binary__exponent_8cpp.html#aeb48dce0725e63d19147944f41843c73">binExpo</a>(a, m - 2, m); <span class="comment">// Fermat's Little Theorem</span></div>
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<div class="line"><span class="lineno"> 114</span>}</div>
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<div class="ttc" id="abinary__exponent_8cpp_html_aeb48dce0725e63d19147944f41843c73"><div class="ttname"><a href="../../de/dcf/binary__exponent_8cpp.html#aeb48dce0725e63d19147944f41843c73">binExpo</a></div><div class="ttdeci">int binExpo(int a, int b)</div><div class="ttdef"><b>Definition</b> <a href="../../de/dcf/binary__exponent_8cpp_source.html#l00028">binary_exponent.cpp:28</a></div></div>
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<h2 class="memtitle"><span class="permalink"><a href="#aa8dca7b867074164d5f45b0f3851269d">◆ </a></span>test()</h2>
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<td class="memname">void test </td>
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<td>(</td>
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<span class="mlabels"><span class="mlabel static">static</span></span> </td>
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<p>Self-test implementation. </p>
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<dl class="section return"><dt>Returns</dt><dd>void </dd></dl>
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<p class="definition">Definition at line <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html#l00122">122</a> of file <a class="el" href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html">modular_inverse_fermat_little_theorem.cpp</a>.</p>
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<div class="fragment"><div class="line"><span class="lineno"> 122</span> {</div>
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<div class="line"><span class="lineno"> 123</span> assert(<a class="code hl_function" href="#ae7e807e02c65c6fffd6162b4c66290c2">math::modular_inverse_fermat::modular_inverse</a>(0, 97) == -1);</div>
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<div class="line"><span class="lineno"> 124</span> assert(<a class="code hl_function" href="#ae7e807e02c65c6fffd6162b4c66290c2">math::modular_inverse_fermat::modular_inverse</a>(15, -2) == -1);</div>
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<div class="line"><span class="lineno"> 125</span> assert(<a class="code hl_function" href="#ae7e807e02c65c6fffd6162b4c66290c2">math::modular_inverse_fermat::modular_inverse</a>(3, 10) == -1);</div>
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<div class="line"><span class="lineno"> 126</span> assert(<a class="code hl_function" href="#ae7e807e02c65c6fffd6162b4c66290c2">math::modular_inverse_fermat::modular_inverse</a>(3, 7) == 5);</div>
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<div class="line"><span class="lineno"> 127</span> assert(<a class="code hl_function" href="#ae7e807e02c65c6fffd6162b4c66290c2">math::modular_inverse_fermat::modular_inverse</a>(1, 101) == 1);</div>
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<div class="line"><span class="lineno"> 128</span> assert(<a class="code hl_function" href="#ae7e807e02c65c6fffd6162b4c66290c2">math::modular_inverse_fermat::modular_inverse</a>(-1337, 285179) == 165519);</div>
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<div class="line"><span class="lineno"> 129</span> assert(<a class="code hl_function" href="#ae7e807e02c65c6fffd6162b4c66290c2">math::modular_inverse_fermat::modular_inverse</a>(123456789, 998244353) == 25170271);</div>
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<div class="line"><span class="lineno"> 130</span> assert(<a class="code hl_function" href="#ae7e807e02c65c6fffd6162b4c66290c2">math::modular_inverse_fermat::modular_inverse</a>(-9876543210, 1000000007) == 784794281);</div>
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<div class="line"><span class="lineno"> 131</span>}</div>
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<div class="ttc" id="amodular__inverse__fermat__little__theorem_8cpp_html_ae7e807e02c65c6fffd6162b4c66290c2"><div class="ttname"><a href="#ae7e807e02c65c6fffd6162b4c66290c2">math::modular_inverse_fermat::modular_inverse</a></div><div class="ttdeci">std::int64_t modular_inverse(std::int64_t a, std::int64_t m)</div><div class="ttdoc">calculates the modular inverse.</div><div class="ttdef"><b>Definition</b> <a href="../../d8/d53/modular__inverse__fermat__little__theorem_8cpp_source.html#l00103">modular_inverse_fermat_little_theorem.cpp:103</a></div></div>
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