From 92ab8a74c96b9e0f684c45aec9cf7f57405ff8da Mon Sep 17 00:00:00 2001
From: Chris Seymour <chris.j.seymour@hotmail.com>
Date: Wed, 8 May 2019 17:18:38 +0100
Subject: [PATCH] #28 add delta zigzag (#29)

---
 include/streamvbyte_zigzag.h | 12 ++++++++++++
 src/streamvbyte_zigzag.c     | 15 +++++++++++++++
 tests/unit.c                 | 12 ++++++++++++
 3 files changed, 39 insertions(+)

diff --git a/include/streamvbyte_zigzag.h b/include/streamvbyte_zigzag.h
index 53e7f07..4e5b370 100644
--- a/include/streamvbyte_zigzag.h
+++ b/include/streamvbyte_zigzag.h
@@ -17,12 +17,24 @@ extern "C" {
  */
 void zigzag_encode(const int32_t * in, uint32_t * out, size_t N);
 
+/**
+ * Convert N signed integers to N unsigned integers, using zigzag
+ * delta encoding.
+ */
+void zigzag_delta_encode(const int32_t * in, uint32_t * out, size_t N, int32_t prev);
+
 /**
  * Convert N unsigned integers to N signed integers, using zigzag
  * encoding.
  */
 void zigzag_decode(const uint32_t * in, int32_t * out, size_t N);
 
+/**
+ * Convert N unsigned integers to N signed integers, using zigzag
+ * delta encoding.
+ */
+void zigzag_delta_decode(const uint32_t * in, int32_t * out, size_t N, int32_t prev);
+
 
 #if defined(__cplusplus)
 };
diff --git a/src/streamvbyte_zigzag.c b/src/streamvbyte_zigzag.c
index cf77cca..dffbecb 100644
--- a/src/streamvbyte_zigzag.c
+++ b/src/streamvbyte_zigzag.c
@@ -10,6 +10,12 @@ void zigzag_encode(const int32_t * in, uint32_t * out, size_t N) {
       out[i] = _zigzag_encode_32(in[i]);
 }
 
+void zigzag_delta_encode(const int32_t * in, uint32_t * out, size_t N, int32_t prev) {
+    for (size_t i = 0; i < N; i++) {
+      out[i] = _zigzag_encode_32(in[i] - prev);
+      prev = in[i];
+    }
+}
 
 static inline
 int32_t _zigzag_decode_32 (uint32_t val) {
@@ -21,3 +27,12 @@ void zigzag_decode(const uint32_t * in, int32_t * out, size_t N) {
     for(size_t i = 0; i < N; i++)
       out[i] = _zigzag_decode_32(in[i]);
 }
+
+
+void zigzag_delta_decode(const uint32_t * in, int32_t * out, size_t N, int32_t prev) {
+    for(size_t i = 0; i < N; i++) {
+      int32_t val =_zigzag_decode_32(in[i]);
+      out[i] = val + prev;
+      prev += val;
+    }
+}
diff --git a/tests/unit.c b/tests/unit.c
index e8aeccc..e71da8a 100644
--- a/tests/unit.c
+++ b/tests/unit.c
@@ -20,15 +20,27 @@ int zigzagtests() {
       datain[i] = rand() - rand();
     uint32_t *dataout = malloc(N * sizeof(uint32_t));
     int32_t *databack = malloc(N * sizeof(int32_t));
+
+    uint32_t *deltadataout = malloc(N * sizeof(uint32_t));
+    int32_t *deltadataback = malloc(N * sizeof(int32_t));
+
     zigzag_encode(datain, dataout, N);
     zigzag_decode(dataout, databack, N);
+    zigzag_delta_encode(datain, deltadataout, N, 0);
+    zigzag_delta_decode(deltadataout, deltadataback, N, 0);
+
     int isok = 1;
     for(size_t i = 0; i < N; i++) {
       if(datain[i] != databack[i]) {
         printf("bug\n");
         isok = -1;
       }
+      if(datain[i] != deltadataback[i]) {
+        printf("bug\n");
+        isok = -1;
+      }
     }
+
     free(databack);
     free(dataout);
     free(datain);