From 6aec103b134f3229e3b0329508c9bd5bd423e880 Mon Sep 17 00:00:00 2001
From: Tomasz Kramkowski <tk@the-tk.com>
Date: Sat, 4 Apr 2015 20:34:17 +0200
Subject: guesskeylength: Calculates indices of coincidence for different key
 width possibilities.

---
 guesskeylength.c | 81 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 81 insertions(+)
 create mode 100644 guesskeylength.c

diff --git a/guesskeylength.c b/guesskeylength.c
new file mode 100644
index 0000000..7941a93
--- /dev/null
+++ b/guesskeylength.c
@@ -0,0 +1,81 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <ctype.h>
+
+#define CTOI(c) ((c) - 'A')
+#define ITOC(c) ((c) + 'A')
+
+#define ALPHACHARS 26
+
+void preprocess(char *str)
+{
+	char *src = str, *dest = str;
+
+	while (*src != '\0') {
+		if (isalpha(*src))
+			*(dest++) = toupper(*src);
+		src++;
+	}
+
+	*dest = '\0';
+}
+
+double calc_ic_column(const char *text, size_t length, size_t offset, size_t spacing)
+{
+	unsigned foundc[ALPHACHARS] = {0}, total_chars = 0;
+	unsigned long pre_sum = 0;
+
+	for (size_t i = offset; i < length; i += spacing, total_chars++)
+		foundc[CTOI(text[i])]++;
+
+	for (unsigned i = 0; i < ALPHACHARS; i++)
+		pre_sum += foundc[i] * (foundc[i] - 1);
+
+	return (double)pre_sum / ((double)total_chars * (double)(total_chars - 1) / ALPHACHARS);
+}
+
+double calc_ic(const char *text, size_t length, size_t width)
+{
+	double total = 0;
+
+	for (size_t offset = 0; offset < width; offset++)
+		total += calc_ic_column(text, length, offset, width);
+
+	return total / width;
+}
+
+int main(int argc, char **argv)
+{
+	char *text;
+	size_t length;
+	unsigned long max;
+
+	if (argc != 3) {
+		fprintf(stderr, "Usage: %s <text> <max>\a\n", argv[0]);
+		exit(EXIT_FAILURE);
+	}
+
+	text = argv[1];
+	max = strtoul(argv[2], NULL, 10);
+
+	preprocess(text);
+	length = strlen(text);
+
+	for (unsigned long spacing = 1; spacing <= max; spacing++) {
+		double ic = calc_ic(text, length, spacing);
+		unsigned barlen;
+
+		if (ic > 0.8 && ic < 4)
+			barlen = (ic - 0.8) / 3.2 * 40.0;
+		else
+			barlen = ic <= 0.8 ? 0 : 40;
+
+		printf("%3lu - %6.3f ", spacing, ic);
+		for (unsigned i = 0; i < barlen; i++)
+			putchar('#');
+		putchar('\n');
+	}
+
+	return EXIT_SUCCESS;
+}
-- 
cgit v1.2.3-54-g00ecf