implement delta compression

src/compress.zig

@@ -6,6 +6,60 @@
 
 const std = @import("std");
 
+// delta compresses an incrementing sorted array of integers using delta
+// compression. Sorting is in-place.
+pub fn deltaCompress(comptime T: type, elems: []T) error{NotSorted}!void {
+    if (elems.len == 0) {
+        return;
+    }
+    var prev: T = elems[0];
+    var i: usize = 1;
+    while (i < elems.len) : (i += 1) {
+        const cur = elems[i];
+        if (cur <= prev) {
+            return error.NotSorted;
+        }
+        elems[i] = cur - prev - 1;
+        prev = cur;
+    }
+}
+
+const testing = std.testing;
+
+test "delta compression positive tests" {
+    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{ 0, 254, 255 }, .want = &[_]u8{ 0, 253, 0 } },
+    };
+    for (tests) |t| {
+        var arr = try std.ArrayList(u8).initCapacity(
+            testing.allocator,
+            t.input.len,
+        );
+        defer arr.deinit();
+        try arr.appendSlice(t.input);
+
+        try deltaCompress(u8, arr.items);
+        try testing.expectEqualSlices(u8, arr.items, t.want);
+    }
+}
+
+test "delta compression negative tests" {
+    for ([_][]const u8{
+        &[_]u8{ 0, 0 },
+        &[_]u8{ 0, 1, 1 },
+        &[_]u8{ 0, 1, 2, 1 },
+    }) |t| {
+        var arr = try std.ArrayList(u8).initCapacity(testing.allocator, t.len);
+        defer arr.deinit();
+        try arr.appendSlice(t);
+        try testing.expectError(error.NotSorted, deltaCompress(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 {
@@ -58,9 +112,8 @@ pub fn putUvarint(buf: []u8, x: u64) usize {
     return i + 1;
 }
 
-const testing = std.testing;
-
-const tests = [_]u64{
+test "uvarint" {
+    const uvarint_tests = [_]u64{
         0,
         1,
         2,
@@ -77,9 +130,7 @@ const tests = [_]u64{
         257,
         1 << 63 - 1,
     };
-
-test "uvarint" {
-    for (tests) |x| {
+    for (uvarint_tests) |x| {
         var buf: [maxVarintLen64]u8 = undefined;
         const n = putUvarint(buf[0..], x);
         const got = try uvarint(buf[0..n]);