add binsearch

.gitignore

@@ -16,3 +16,4 @@ multibrot.png
 bloom
 wo-lernen
 lua-foo
+binsearch

Makefile

@@ -1,7 +1,7 @@
 CFLAGS=-std=c99 -Wall -g -O2
 INDENTOPTS=-kr --no-tabs --braces-on-func-def-line --indent-level2
 
-TARGETS=approximate-pi linked-list mandelbrot threads circular-buffer structs ncurses-pong bit-fuckery bit-fuckery2 checkcheck multibrot bloom wo-lernen lua-foo
+TARGETS=approximate-pi linked-list mandelbrot threads circular-buffer structs ncurses-pong bit-fuckery bit-fuckery2 checkcheck multibrot bloom wo-lernen lua-foo binsearch
 EXTRAS=mandelbrot.bmp multibrot.png
 
 .PHONY: all

binsearch.c

@@ -0,0 +1,80 @@
+/*
+Write a recursive function that searches for a target in a sorted array using
+binay search, where the array, its size and the target are given as parameters.
+*/
+
+#include <limits.h>
+#include <stdio.h>
+#include <assert.h>
+
+#define DEBUG 1
+#define debug_printf(fmt, ...) \
+            do { if (DEBUG) fprintf(stderr, fmt, __VA_ARGS__); } while (0)
+
+void assert_sorted(int array[], int arraylen) {
+  int n = INT_MIN;
+
+  for (int i = 0; i < arraylen; i++) {
+    assert(n <= array[i]);
+    n = array[i];
+  }
+}
+
+int binsearchr(int array[], int from, int to, int target) {
+  int m = from + (to-from)/2;
+  debug_printf("from = %d, to = %d, m = %d\n", from, to, m);
+
+  if (to < from)
+    return -1;
+
+  if (array[m] == target) {
+    return m;
+  } if (array[m] > target) {
+    return binsearchr(array, from, m-1, target);
+  } else {
+    /* array[m] < target */
+    return binsearchr(array, m+1,  to, target);
+  }
+}
+
+int binsearch(int array[], int arraylen, int target) {
+  return binsearchr(array, 0, arraylen-1, target);
+}
+
+int main(void) {
+  int a[43] = {
+    912, 1204, 1761, 2143, 3691,
+    5150, 5249, 6864, 6871, 6902,
+    9567, 9703, 10411, 10686, 13501,
+    14378, 16249, 17393, 17911, 18614,
+    19284, 19595, 19960, 20052, 20702,
+    21090, 22277, 22624, 24634, 24877,
+    25367, 25586, 26706, 26720, 26958,
+    27478, 28269, 29142, 30630, 30804,
+    31984, 32398, 32588 };
+  assert_sorted(a, 43);
+
+  assert(binsearch(a, 43, 912) == 0);
+  assert(binsearch(a, 43, 3691) == 4);
+  assert(binsearch(a, 43, 32588) == 42);
+  assert(binsearch(a, 43, 12345) == -1);
+
+  int b[2] = { 23, 42 };
+  assert_sorted(b, 2);
+
+  assert(binsearch(b, 2, 23) == 0);
+  assert(binsearch(b, 2, 42) == 1);
+  assert(binsearch(b, 2, 5) == -1);
+  assert(binsearch(b, 2, 99) == -1);
+
+  int c[1] = { 0 };
+  assert_sorted(c, 1);
+
+  assert(binsearch(c, 1, 0) == 0);
+  assert(binsearch(c, 1, 1) == -1);
+
+  int d[0];
+  assert_sorted(d, 1);
+
+  assert(binsearch(d, 0, 0) == -1);
+}