created why2_checksum_segment fn

it isn't meant to be unique checksum. it just really messses with the range of values

src/core/lib/utils/crypto.c

@@ -0,0 +1,45 @@
+/*
+This is part of WHY2
+Copyright (C) 2022 Václav Šmejkal
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.
+*/
+
+#include <why2/crypto.h>
+
+#include <string.h>
+#include <math.h>
+
+unsigned long why2_checksum_segment(char *input)
+{
+    unsigned long input_size = strlen(input);
+    unsigned long segmented_input_size = ceil(input_size / (double) WHY2_CHECKSUM_SEGMENT_SIZE) * WHY2_CHECKSUM_SEGMENT_SIZE; //CALCULATE CLOSEST 4*n TO input
+    unsigned long output = 0;
+
+    for (unsigned long i = 0; i < segmented_input_size / WHY2_CHECKSUM_SEGMENT_SIZE; i++) //DIVIDE buffer INTO SEGMENTS, XOR EACH OTHER AND ADD TO output
+    {
+        unsigned long output_buffer = 0;
+        for (unsigned long j = 0; j < WHY2_CHECKSUM_SEGMENT_SIZE; j++)
+        {
+            unsigned long index_buffer = i * WHY2_CHECKSUM_SEGMENT_SIZE + j;
+
+            output_buffer ^= (input_size > index_buffer) ? input[index_buffer] : 0; //XORING
+            output_buffer = (output_buffer << WHY2_CHECKSUM_PRIME) | (output_buffer >> (sizeof(unsigned long) * 8 - WHY2_CHECKSUM_PRIME)); //ROTATE
+        }
+
+        output += output_buffer; //ADD
+    }
+
+    return output;
+}