first commit

main.c

@@ -0,0 +1,79 @@
+#include <stdio.h>
+#include <stdint.h>
+
+// This is the length of each block containig the binary rappresentation of the occurrence for each digit
+// Up to 6 bits (because 10 digits * 6 is the nearest to 64 which is the max size of uint64_t)
+// this means also (6 bit) 111111 or (decimal) 63 MAX rep for each number 0-9
+#define BITS_LENGTH 6
+
+// Create a mask with the same length of the uint64_t (64 bit) with all 1s inside.
+#define ALL_ONES_MASK ((uint64_t)1 << 63) - 1 
+
+
+
+// Bit container
+uint64_t rep = (uint64_t)0;
+
+/*
+    Get the occurrence of a digit
+*/
+unsigned getRep(unsigned n) {
+    // Shift the bits of the digit n to the LSP and use a bitwise & to isolate the bits we need. 
+    return (rep >> n*BITS_LENGTH) & ((1 << BITS_LENGTH) - 1);
+}
+
+
+/*
+    Increase the number of times n is repeated.
+*/
+void incr(unsigned n) {
+    uint64_t mask, nrep;
+
+    nrep = getRep(n); // occurrence of specif digit.
+
+    // Create a pattern of all 1s of BITS_LENGTH length. 
+    // then shift it to the left in the bit position rappresenting the digit we need to count.
+    uint64_t pattern1s = (((uint64_t)1 << BITS_LENGTH) - 1 ) << n*BITS_LENGTH;
+
+    // then subract pattern1s from the ALL_ONES_MASK mask, obtaining a 
+    // new mask that we'll use to bitwise AND rep to update the new repetition.
+    mask = ALL_ONES_MASK - pattern1s;
+
+    // first part resets the 4 bits block rappresenting the occurrence of the digit to 4 zeros.
+    // then bitwise OR the result with the increment + 1 of nrep (rep of that digit) 
+    // shifted to the left in the position of the 4 zeros bits spot created before.
+
+    // in this way we can increment it without altering the others digits occurrence.
+    rep = rep & mask | (nrep + 1 << BITS_LENGTH*n);
+
+}
+
+
+
+int main()
+{
+    unsigned current_rep = 0;
+    int digit;
+
+    printf("Input number: ");
+    while (1) {
+        digit = getchar();
+        if (digit != '\n' && (digit >= '0' && digit <= '9')) {
+            incr(digit - '0');
+        }
+        else
+            break;
+    }
+
+    printf("\n"); // just a space
+
+    for (unsigned i = 0; i < 10; i++) {
+        current_rep = getRep(i);
+        if (current_rep > 1) {
+            printf("Occurrence of %u: %u\n", i, current_rep);
+        }
+    }
+
+
+    return 0;
+}