1 files changed, 108 insertions, 0 deletions
diff --git a/kattis-kth/alginda-quicksort/radix.c b/kattis-kth/alginda-quicksort/radix.c
new file mode 100644
index 0000000..93bd111
--- /dev/null
+++ b/kattis-kth/alginda-quicksort/radix.c
@@ -0,0 +1,108 @@
+/* https://kth.kattis.com/problems/kth.alginda.quicksort */
+
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <unistd.h>
+
+// longest int: -2147483648
+#define BUFFER_MAX (12 * 600000)
+char buffer[BUFFER_MAX];
+int xs[600000];
+int tmp[600000];
+
+void radix_sort(int n) {
+    int *out = xs;
+    int *in = tmp;
+    // we loop an even amount of times so the final sorted array will be in xs.
+    // if we looped an odd amount of times this would not be the case
+    for (int m = 0; m < 32; m += 8) {
+        int *tmp = out;
+        out = in;
+        in = tmp;
+
+        int counts[256] = { 0 };
+        for (int i = 0; i < n; i++)
+            counts[(uint8_t) (in[i] >> m)]++;
+
+        int sum = 0;
+        for (int i = 0; i < 256; i++) {
+            sum += counts[i];
+            counts[i] = sum;
+        }
+        for (int i = n - 1; i >= 0; i--) {
+            uint8_t fx = in[i] >> m;
+            counts[fx]--;
+            out[counts[fx]] = in[i];
+        }
+    }
+}
+
+int main() {
+    /* char *buffer = malloc(BUFFER_MAX); */
+    /* ssize_t len = 0; */
+    /* while (true) { */
+    /*     if (len >= BUFFER_MAX) return 1; */
+    /*     ssize_t r = read(0, &buffer[len], BUFFER_MAX - len); */
+    /*     len += r; */
+    /*     if (r == 0) */
+    /*         break; */
+    /* } */
+    ssize_t len = read(0, buffer, BUFFER_MAX);
+
+    char *p = buffer;
+    char *end = &buffer[len];
+    int n = 0;
+    while (*p != ' ') {
+        n *= 10;
+        n += *p - '0';
+        p++;
+    }
+    while (*p == ' ' || *p == '\n') p++;
+
+    // 18 ms
+    for (int i = 0; i < n; i++) {
+        int x = 0;
+        bool neg = false;
+        if (*p == '-') {
+            neg = true;
+            p++;
+        }
+        while (p < end && *p != ' ' && *p != '\n') {
+            x *= 10;
+            x += *p - '0';
+            p++;
+        }
+        while (p < end && (*p == ' ' || *p == '\n')) p++;
+
+        if (neg) x = -x;
+        xs[i] = x ^ (1 << 31);
+    }
+
+    // 10 ms
+    radix_sort(n);
+
+    // 17 ms
+    p = &buffer[BUFFER_MAX - 1];
+    char *last = p;
+    *p-- = '\n';
+    for (int i = n - 1; i >= 0; i--) {
+        int signedx = xs[i] ^ (1 << 31);
+        bool neg = signedx < 0;
+        uint32_t x;
+        if (neg) x = -signedx;
+        else     x = signedx;
+        do {
+            *p-- = (x % 10) + '0';
+            x /= 10;
+        } while (x > 0);
+        if (neg) *p-- = '-';
+        if (i > 0) *p-- = '\n';
+    }
+    write(1, p + 1, last - p);
+
+    /* ssize_t c = 0; */
+    /* while (c < len) c += write(1, &buffer[c], len - c); */
+}