solve day 6

src/day5.zig

@@ -1,6 +1,5 @@
 const aoc = @import("./aoc.zig");
 const std = @import("std");
-const expect = std.testing.expect;
 
 const MappingRange = struct {
     src_start: u64,

src/day6.zig

@@ -0,0 +1,115 @@
+const std = @import("std");
+const aoc = @import("./aoc.zig");
+const assert = std.debug.assert;
+
+const BoundedU32 = std.BoundedArray(u32, 32);
+
+const Input = struct {
+    time: BoundedU32,
+    distance: BoundedU32,
+};
+
+fn parse_numbers(result: *BoundedU32, text: []const u8) !void {
+    var i: usize = 0;
+    var tokens = std.mem.tokenizeScalar(u8, text, ' ');
+    while (tokens.next()) |token| : (i += 1) {
+        const number = try std.fmt.parseInt(u32, token, 10);
+        try result.append(number);
+    }
+}
+
+fn parse_input(lines: []const []const u8) !Input {
+    var time = try BoundedU32.init(0);
+    var distance = try BoundedU32.init(0);
+    try parse_numbers(&time, lines[0][9..]);
+    try parse_numbers(&distance, lines[1][9..]);
+    assert(time.len == distance.len);
+    return Input{ .time = time, .distance = distance };
+}
+
+pub fn part1(input: *aoc.Input) !aoc.Result {
+    var parsed = try parse_input(input.lines);
+
+    var answer: u32 = 1;
+    for (0..parsed.time.len) |i| {
+        const required_distance = parsed.distance.get(i);
+        const max_allowed_time = parsed.time.get(i);
+
+        var min_time: u32 = 0;
+        var max_time: u32 = 0;
+        for (1..(max_allowed_time+1)) |speed_usize| {
+            const speed: u32 = @intCast(speed_usize);
+            const distance = speed*(max_allowed_time - speed);
+            if (distance <= required_distance) continue;
+
+            if (max_time == 0) {
+                min_time = speed;
+                max_time = speed;
+            } else {
+                max_time = @max(max_time, speed);
+            }
+        }
+
+        const margin = max_time - min_time + 1;
+        answer *= margin;
+    }
+
+    return .{ .uint = answer };
+}
+
+pub fn part2(input: *aoc.Input) !aoc.Result {
+    var parsed = try parse_input(input.lines);
+
+    var allowed_time: u64 = 0;
+    var required_distance: u64 = 0;
+    for (0..parsed.time.len) |i| {
+        const time = parsed.time.get(i);
+        if (allowed_time > 0) {
+            allowed_time *= try std.math.powi(u32, 10, std.math.log10(time)+1);
+        }
+        allowed_time += time;
+
+        const distance = parsed.distance.get(i);
+        if (required_distance > 0) {
+            required_distance *= try std.math.powi(u32, 10, std.math.log10(distance)+1);
+        }
+        required_distance += distance;
+    }
+
+    var min_time: u64 = 0;
+    var max_time: u64 = 0;
+    for (1..(allowed_time+1)) |speed_usize| {
+        const speed: u64 = @intCast(speed_usize);
+        const distance = speed*(allowed_time - speed);
+        if (distance <= required_distance) continue;
+
+        if (max_time == 0) {
+            min_time = speed;
+            max_time = speed;
+        } else {
+            max_time = @max(max_time, speed);
+        }
+    }
+
+    var answer = max_time - min_time + 1;
+    return .{ .uint = @intCast(answer) };
+}
+
+const example_input = [_][]const u8{
+    "Time:      7  15   30",
+    "Distance:  9  40  200"
+};
+
+test "part 1 example" {
+    var input = aoc.Input{ .lines = &example_input, .allocator = std.testing.allocator };
+    var actual = try part1(&input);
+    var expected: u32 = 288;
+    try std.testing.expectEqual(expected, actual.uint);
+}
+
+test "part 2 example" {
+    var input = aoc.Input{ .lines = &example_input, .allocator = std.testing.allocator };
+    var actual = try part2(&input);
+    var expected: u32 = 71503;
+    try std.testing.expectEqual(expected, actual.uint);
+}

src/main.zig

@@ -26,6 +26,7 @@ const Days = [_]aoc.Day{
     create_day(@import("day3.zig")),
     create_day(@import("day4.zig")),
     create_day(@import("day5.zig")),
+    create_day(@import("day6.zig")),
 };
 
 fn kilobytes(count: u32) u32 {
@@ -172,4 +173,5 @@ test {
     _ = @import("day3.zig");
     _ = @import("day4.zig");
     _ = @import("day5.zig");
+    _ = @import("day6.zig");
 }