Blog Entry (c) Sunday, February 22, 2026, by James Pate Williams, Jr. A Little Test C Program Source Code and Results

// FIPS202Algorithm10.c
// James Pate Williams, Jr.
// Copyright Sunday, February
// 22, 2026 See FIPS 202
// Algorithm 10

#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

char gHexStr[2048];
int gBinaryStr[8];
int gH[2048], gh[1024];
int gT[8 * 1024 + 7];
int gTruncN[1024];
int gS[1024];

static void ToBinary(int hex)
{
    int count = 0;

    while (hex > 0)
    {
        gBinaryStr[count++] = hex & 1;
        hex >>= 1;
    }

    for (int i = count; i < 8; i++)
        gBinaryStr[i] = 0;
}

static void Trunc(int X[], int n)
{
    for (int i = 0; i < n; i++)
        gTruncN[i] = X[i];
}

static void Algorithm10(int m, int n)
{
    for (int i = 0; i < m; i++)
    {
        gh[i] = 16 * gH[2 * i] + gH[2 * i + 1];
    }

    for (int i = 0; i < m; i++)
    {
        ToBinary(gh[i]);

        for (int j = 0; j < 8; j++)
            gT[8 * i + j] = gBinaryStr[j];
    }

    Trunc(gT, n);

    for (int i = 0; i < n; i++)
        gS[i] = gTruncN[i];
}

static void HexToBinary(int m, int n)
{
    int m2 = m + m;

    printf_s("H = ");

    for (int i = 0; i < m2; i++)
    {
        int digit = 0;

        if (gHexStr[i] >= '0' && gHexStr[i] <= '9')
            digit = gHexStr[i] - '0';
        else if (gHexStr[i] >= 'a' && gHexStr[i] <= 'f')
            digit = gHexStr[i] - 'a' + 10;
        else if (gHexStr[i] >= 'A' && gHexStr[i] <= 'F')
            digit = gHexStr[i] - 'A' + 10;

        gH[i] = digit;

        printf_s("% d ", gH[i]);
    }

    printf_s("\r\n");
    Algorithm10(m, n);
}

int main()
{
    int n = 0;
    printf_s("Enter a hex string\r\n");
    scanf_s("%s", gHexStr, 128);
    printf_s("Enter n\r\n");
    scanf_s("%d", &n);
    int m = (int)strlen(gHexStr);

    HexToBinary(m / 2, n);

    printf_s("S = ");

    for (int i = 0; i < n; i++)
        printf_s("%d ", gS[i]);

    printf_s("\r\n");
    return 0;
}