varint iterator

src/compress.zig

@@ -7,6 +7,7 @@ const std = @import("std");
 
 const ArrayList = std.ArrayList;
 const Allocator = std.mem.Allocator;
+const assert = std.debug.assert;
 
 // compresses a strictly incrementing sorted slice of integers using delta
 // compression. Compression is in-place.
@@ -39,6 +40,93 @@ pub fn deltaDecompress(comptime T: type, elems: []T) error{Overflow}!void {
     }
 }
 
+// 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 {
+    value: u64,
+    bytes_read: usize,
+};
+
+pub const maxVarintLen64 = 10;
+
+// https://golang.org/pkg/encoding/binary/#Uvarint
+pub fn uvarint(buf: []const u8) error{Overflow}!Varint {
+    var x: u64 = 0;
+    var s: u6 = 0;
+
+    for (buf) |b, i| {
+        if (i == maxVarintLen64)
+            // Catch byte reads past maxVarintLen64.
+            // See issue https://golang.org/issues/41185
+            return error.Overflow;
+
+        if (b < 0x80) {
+            if (i == maxVarintLen64 - 1 and b > 1) {
+                return error.Overflow;
+            }
+            return Varint{
+                .value = x | (@as(u64, b) << s),
+                .bytes_read = i + 1,
+            };
+        }
+        x |= (@as(u64, b & 0x7f) << s);
+        s = try std.math.add(u6, s, 7);
+    }
+
+    return Varint{
+        .value = 0,
+        .bytes_read = 0,
+    };
+}
+
+// https://golang.org/pkg/encoding/binary/#PutUvarint
+pub fn putUvarint(buf: []u8, x: u64) usize {
+    var i: usize = 0;
+    var mutX = x;
+
+    while (mutX >= 0x80) {
+        buf[i] = @truncate(u8, mutX) | 0x80;
+        mutX >>= 7;
+        i += 1;
+    }
+    buf[i] = @truncate(u8, mutX);
+
+    return i + 1;
+}
+
+// VarintSliceIterator iterates over varint-encoded slice.
+// The first element is the length of the slice, in decoded numbers.
+const varintSliceIterator = struct {
+    remaining: usize,
+    arr: []const u8,
+    idx: usize,
+
+    pub fn next(it: *varintSliceIterator) error{Overflow}!?u64 {
+        if (it.remaining == 0)
+            return null;
+        const value = try uvarint(it.arr[it.idx..]);
+        it.idx += value.bytes_read;
+        it.remaining -= 1;
+        return value.value;
+    }
+};
+
+pub fn VarintSliceIterator(arr: []const u8) error{Overflow}!varintSliceIterator {
+    const firstnumber = try uvarint(arr);
+
+    return varintSliceIterator{
+        .remaining = firstnumber.value,
+        .arr = arr,
+        .idx = firstnumber.bytes_read,
+    };
+}
+
+pub fn appendUvarint(arr: *ArrayList(u8), x: u64) Allocator.Error!void {
+    var buf: [maxVarintLen64]u8 = undefined;
+    const n = putUvarint(&buf, x);
+    try arr.appendSlice(buf[0..n]);
+}
+
 const testing = std.testing;
 
 test "delta compress/decompress" {
@@ -91,66 +179,6 @@ test "delta decompress overflow" {
     }
 }
 
-// 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 {
-    value: u64,
-    bytesRead: usize,
-};
-
-pub const maxVarintLen64 = 10;
-
-// https://golang.org/pkg/encoding/binary/#Uvarint
-pub fn uvarint(buf: []const u8) error{Overflow}!Varint {
-    var x: u64 = 0;
-    var s: u6 = 0;
-
-    for (buf) |b, i| {
-        if (i == maxVarintLen64)
-            // Catch byte reads past maxVarintLen64.
-            // See issue https://golang.org/issues/41185
-            return error.Overflow;
-
-        if (b < 0x80) {
-            if (i == maxVarintLen64 - 1 and b > 1) {
-                return error.Overflow;
-            }
-            return Varint{
-                .value = x | (@as(u64, b) << s),
-                .bytesRead = i + 1,
-            };
-        }
-        x |= (@as(u64, b & 0x7f) << s);
-        s = try std.math.add(u6, s, 7);
-    }
-
-    return Varint{
-        .value = 0,
-        .bytesRead = 0,
-    };
-}
-
-// https://golang.org/pkg/encoding/binary/#PutUvarint
-pub fn putUvarint(buf: []u8, x: u64) usize {
-    var i: usize = 0;
-    var mutX = x;
-
-    while (mutX >= 0x80) {
-        buf[i] = @truncate(u8, mutX) | 0x80;
-        mutX >>= 7;
-        i += 1;
-    }
-    buf[i] = @truncate(u8, mutX);
-
-    return i + 1;
-}
-
-pub fn appendUvarint(arr: *ArrayList(u8), x: u64) Allocator.Error!void {
-    var buf: [maxVarintLen64]u8 = undefined;
-    const n = putUvarint(&buf, x);
-    try arr.appendSlice(buf[0..n]);
-}
-
 const uvarint_tests = [_]u64{
     0,
     1,
@@ -176,10 +204,25 @@ test "putUvarint/uvarint" {
         const got = try uvarint(buf[0..n]);
 
         try testing.expectEqual(x, got.value);
-        try testing.expectEqual(n, got.bytesRead);
+        try testing.expectEqual(n, got.bytes_read);
     }
 }
 
+test "VarintSliceIterator" {
+    var buf = ArrayList(u8).init(testing.allocator);
+    defer buf.deinit();
+    try appendUvarint(&buf, uvarint_tests.len);
+    for (uvarint_tests) |x|
+        try appendUvarint(&buf, x);
+
+    var it = try VarintSliceIterator(buf.items);
+    var i: usize = 0;
+    while (try it.next()) |got| : (i += 1) {
+        try testing.expectEqual(uvarint_tests[i], got);
+    }
+    try testing.expectEqual(i, uvarint_tests.len);
+}
+
 test "appendUvarint" {
     for (uvarint_tests) |x| {
         var buf = ArrayList(u8).init(testing.allocator);

src/sections.zig

@@ -263,13 +263,15 @@ pub const usersSectionErr = Allocator.Error || error{Overflow};
 pub fn usersSection(
     allocator: Allocator,
     corpus: *const Corpus,
-    gids: UserGids,
+    gids: *const UserGids,
-    shells: ShellSections,
+    shells: *const ShellSections,
 ) usersSectionErr![]const u8 {
-    _ = allocator;
+    // as of writing each user takes 15 bytes + strings + padding, padded to
-    _ = corpus;
+    // 8 bytes. 24 is a very optimistic lower bound.
+    var buf = try ArrayList(u8).initCapacity(allocator, 24 * corpus.users.len);
     _ = gids;
     _ = shells;
+    return buf.toOwnedSlice();
 }
 
 pub fn groupMembers(allocator: Allocator, corpus: *const Corpus) Allocator.Error!void {
@@ -447,6 +449,23 @@ test "test sections" {
 
     var user_gids = try userGids(allocator, &corpus);
     defer user_gids.deinit(allocator);
+
+    var users_section = try usersSection(
+        allocator,
+        &corpus,
+        &user_gids,
+        &shell_sections,
+    );
+    defer allocator.free(users_section);
+}
+
+test "userGids" {
+    const allocator = testing.allocator;
+    var corpus = try testCorpus(allocator);
+    defer corpus.deinit();
+
+    var user_gids = try userGids(allocator, &corpus);
+    defer user_gids.deinit(allocator);
 }
 
 test "pack gids" {