From 0b0e917086fd5a1cc72badafbb7ce55c5ff445a7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Motiejus=20Jak=C5=A1tys?= <motiejus@jakstys.lt>
Date: Mon, 28 Feb 2022 06:02:16 +0200
Subject: [PATCH] deltaDecompress

---
 src/compress.zig | 37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/src/compress.zig b/src/compress.zig
index cfac66b..7368251 100644
--- a/src/compress.zig
+++ b/src/compress.zig
@@ -6,10 +6,10 @@
 
 const std = @import("std");
 
-// delta compresses an incrementing sorted array of integers using delta
-// compression. Sorting is in-place.
+// compresses a strictly incrementing sorted slice of integers using delta
+// compression. Compression is in-place.
 pub fn deltaCompress(comptime T: type, elems: []T) error{NotSorted}!void {
-    if (elems.len == 0) {
+    if (elems.len <= 1) {
         return;
     }
     var prev: T = elems[0];
@@ -24,14 +24,28 @@ pub fn deltaCompress(comptime T: type, elems: []T) error{NotSorted}!void {
     }
 }
 
+// decompresses a slice compressed by deltaCompress. In-place.
+pub fn deltaDecompress(comptime T: type, elems: []T) error{Overflow}!void {
+    if (elems.len <= 1) {
+        return;
+    }
+
+    var i: usize = 1;
+    while (i < elems.len) : (i += 1) {
+        const x = try std.math.add(T, elems[i - 1], 1);
+        elems[i] = try std.math.add(T, elems[i], x);
+    }
+}
+
 const testing = std.testing;
 
-test "delta compression positive tests" {
+test "delta compress/decompress" {
     const tests = [_]struct { input: []const u8, want: []const u8 }{
         .{ .input = &[_]u8{}, .want = &[_]u8{} },
         .{ .input = &[_]u8{0}, .want = &[_]u8{0} },
         .{ .input = &[_]u8{10}, .want = &[_]u8{10} },
         .{ .input = &[_]u8{ 0, 1, 2 }, .want = &[_]u8{ 0, 0, 0 } },
+        .{ .input = &[_]u8{ 10, 20, 30, 255 }, .want = &[_]u8{ 10, 9, 9, 224 } },
         .{ .input = &[_]u8{ 0, 254, 255 }, .want = &[_]u8{ 0, 253, 0 } },
     };
     for (tests) |t| {
@@ -44,6 +58,9 @@ test "delta compression positive tests" {
 
         try deltaCompress(u8, arr.items);
         try testing.expectEqualSlices(u8, arr.items, t.want);
+
+        try deltaDecompress(u8, arr.items);
+        try testing.expectEqualSlices(u8, arr.items, t.input);
     }
 }
 
@@ -60,6 +77,18 @@ test "delta compression negative tests" {
     }
 }
 
+test "delta decompress overflow" {
+    for ([_][]const u8{
+        &[_]u8{ 255, 0 },
+        &[_]u8{ 0, 128, 127 },
+    }) |t| {
+        var arr = try std.ArrayList(u8).initCapacity(testing.allocator, t.len);
+        defer arr.deinit();
+        try arr.appendSlice(t);
+        try testing.expectError(error.Overflow, deltaDecompress(u8, arr.items));
+    }
+}
+
 // Represents a variable length integer that we read from a byte stream along
 // with how many bytes were read to decode it.
 pub const Varint = struct {