implementation for a ziplist with push and pop support

ziplist.c

+/* Memory layout of a ziplist, containing "foo", "bar", "quux":
+ * <zlbytes><zllen><len>"foo"<len>"bar"<len>"quux"
+ *
+ * <zlbytes> is an unsigned integer to hold the number of bytes that
+ * the ziplist occupies. This is stored to not have to traverse the ziplist
+ * to know the new length when pushing.
+ *
+ * <zllen> is the number of items in the ziplist. When this value is
+ * greater than 254, we need to traverse the entire list to know
+ * how many items it holds.
+ *
+ * <len> is the number of bytes occupied by a single entry. When this
+ * number is greater than 253, the length will occupy 5 bytes, where
+ * the extra bytes contain an unsigned integer to hold the length.
+ */
+
+#include <stdio.h>
+#include <string.h>
+#include <assert.h>
+#include "zmalloc.h"
+#include "sds.h"
+#include "ziplist.h"
+#include "zip.c"
+
+#define ZIPLIST_BYTES(zl) (*((unsigned int*)(zl)))
+#define ZIPLIST_LENGTH(zl) (*((zl)+sizeof(unsigned int)))
+#define ZIPLIST_HEADER_SIZE (sizeof(unsigned int)+1)
+
+/* Create a new empty ziplist. */
+unsigned char *ziplistNew(void) {
+    unsigned int bytes = ZIPLIST_HEADER_SIZE+1;
+    unsigned char *zl = zmalloc(bytes);
+    ZIPLIST_BYTES(zl) = bytes;
+    ZIPLIST_LENGTH(zl) = 0;
+    zl[bytes-1] = ZIP_END;
+    return zl;
+}
+
+static unsigned char *ziplistResize(unsigned char *zl, unsigned int len) {
+    zl = zipResize(zl,len);
+    ZIPLIST_BYTES(zl) = len;
+    zl[len-1] = ZIP_END;
+    return zl;
+}
+
+static unsigned char *ziplistHead(unsigned char *zl) {
+    return zl+ZIPLIST_HEADER_SIZE;
+}
+
+static unsigned char *ziplistTail(unsigned char *zl) {
+    unsigned char *p, *q;
+    p = q = ziplistHead(zl);
+    while (*p != ZIP_END) {
+        q = p;
+        p += zipRawEntryLength(p);
+    }
+    return q;
+}
+
+unsigned char *ziplistPush(unsigned char *zl, unsigned char *entry, unsigned int elen, int where) {
+    unsigned int curlen = ZIPLIST_BYTES(zl);
+    unsigned int reqlen = zipEncodeLength(NULL,elen)+elen;
+    unsigned char *p;
+
+    /* Resize the ziplist */
+    zl = ziplistResize(zl,curlen+reqlen);
+
+    if (where == ZIPLIST_HEAD) {
+        p = zl+ZIPLIST_HEADER_SIZE;
+        if (*p != ZIP_END) {
+            /* Subtract one because of the ZIP_END bytes */
+            memmove(p+reqlen,p,curlen-ZIPLIST_HEADER_SIZE-1);
+        }
+    } else {
+        p = zl+curlen-1;
+    }
+
+    /* Increase length */
+    if (ZIPLIST_LENGTH(zl) < ZIP_BIGLEN) ZIPLIST_LENGTH(zl)++;
+
+    /* Write the entry */
+    p += zipEncodeLength(p,elen);
+    memcpy(p,entry,elen);
+    return zl;
+}
+
+unsigned char *ziplistPop(unsigned char *zl, sds *value, int where) {
+    unsigned int curlen = ZIPLIST_BYTES(zl), len, rlen;
+    unsigned char *p;
+    *value = NULL;
+
+    /* Get pointer to element to remove */
+    p = (where == ZIPLIST_HEAD) ? ziplistHead(zl) : ziplistTail(zl);
+    if (*p == ZIP_END) return zl;
+    len = zipDecodeLength(p);
+    *value = sdsnewlen(p+zipEncodeLength(NULL,len),len);
+
+    /* Move list to front when popping from the head */
+    rlen = zipRawEntryLength(p);
+    if (where == ZIPLIST_HEAD) {
+        memmove(p,p+rlen,curlen-ZIPLIST_HEADER_SIZE-len);
+    }
+
+    /* Resize and update length */
+    zl = ziplistResize(zl,curlen-rlen);
+    if (ZIPLIST_LENGTH(zl) < ZIP_BIGLEN) ZIPLIST_LENGTH(zl)--;
+    return zl;
+}
+
+void ziplistRepr(unsigned char *zl) {
+    unsigned char *p;
+    unsigned int l;
+
+    printf("{bytes %d} {length %u}\n",ZIPLIST_BYTES(zl), ZIPLIST_LENGTH(zl));
+    p = ziplistHead(zl);
+    while(*p != ZIP_END) {
+        l = zipDecodeLength(p);
+        printf("{key %u}",l);
+        p += zipEncodeLength(NULL,l);
+        fwrite(p,l,1,stdout);
+        printf("\n");
+        p += l;
+    }
+    printf("{end}\n\n");
+}
+
+#ifdef ZIPLIST_TEST_MAIN
+int main(int argc, char **argv) {
+    unsigned char *zl;
+    sds s;
+
+    zl = ziplistNew();
+    zl = ziplistPush(zl, (unsigned char*)"foo", 3, ZIPLIST_TAIL);
+    ziplistRepr(zl);
+    zl = ziplistPush(zl, (unsigned char*)"quux", 4, ZIPLIST_TAIL);
+    ziplistRepr(zl);
+    zl = ziplistPush(zl, (unsigned char*)"hello", 5, ZIPLIST_HEAD);
+    ziplistRepr(zl);
+
+    zl = ziplistPop(zl, &s, ZIPLIST_TAIL);
+    printf("Pop tail: %s (length %ld)\n", s, sdslen(s));
+    ziplistRepr(zl);
+
+    zl = ziplistPop(zl, &s, ZIPLIST_HEAD);
+    printf("Pop head: %s (length %ld)\n", s, sdslen(s));
+    ziplistRepr(zl);
+
+    return 0;
+}
+#endif

ziplist.h

+#define ZIPLIST_HEAD 0
+#define ZIPLIST_TAIL 1