hash: add hash_digestcb() function

May be useful for calculating hash of a stream of input data.

src/hash.c

@@ -67,6 +67,29 @@ hash_digestv (
     return result;
 }
 
+uint64_t
+hash_digestcb (
+    hash_digestcb_func *callback,
+    void               *user_data
+) {
+    XXH3_state_t *state = XXH3_createState();
+    xassert(state != NULL);
+
+    xassert(0 == XXH3_64bits_reset_withSeed(state, seed));
+    while (1) {
+        void const *buf;
+        size_t data_len = callback(user_data, &buf);
+        if (data_len == 0) {
+            break;
+        }
+        xassert(0 == XXH3_64bits_update(state, buf, data_len));
+    }
+
+    uint64_t result = XXH3_64bits_digest(state);
+    XXH3_freeState(state);
+    return result;
+}
+
 void
 hash_seed (
     uint64_t s

src/hash.h

@@ -29,6 +29,19 @@
 
 #include <sys/uio.h>
 
+/**
+ * Callback function for hash_digestcb().
+ *
+ * The function should store the pointer to a buffer to buf_ptr,
+ * which only need to be valid until the next call to the callback.
+ *
+ * Returns the length of data in buffer, or 0 to terminate.
+ */
+typedef size_t (hash_digestcb_func) (
+    void        *user_data,
+    void const **buf_ptr
+);
+
 /**
  * Calculate the 64-bit hash for the given input.
  */
@@ -49,6 +62,18 @@ hash_digestv (
     int                 bufcnt
 );
 
+/**
+ * Like hash_digest(), but takes a callback function.
+ *
+ * The callback function is called continuously for each part of
+ * the input data, until it returns 0.
+ */
+uint64_t
+hash_digestcb (
+    hash_digestcb_func *callback,
+    void               *user_data
+);
+
 /**
  * Seed the hash function with the given value.
  * If not called, the seed equals to value 0.