80 lines
2.6 KiB
Zig
80 lines
2.6 KiB
Zig
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const std = @import("std");
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const v2d = [2]i32;
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pub fn main() void {
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const input = @embedFile("input");
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std.debug.print("Part 1: {}\n", .{nVisitedSnake(input, 2)});
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std.debug.print("Part 2: {}\n", .{nVisitedSnake(input, 10)});
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}
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pub fn nVisitedSnake(input: []const u8, comptime len: usize) usize {
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if (len < 2) @compileError("The length has to be greater than 1.");
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var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
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defer arena.deinit();
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const allocator = arena.allocator();
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var knots = [_]v2d{v2d{ 0, 0 }} ** len;
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var visited = std.ArrayList(v2d).init(allocator);
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var iter = std.mem.split(u8, input, "\n");
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while (iter.next()) |line| {
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// Line not valid
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if (line.len < 2) continue;
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// Get the number, continue on error
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const num = std.fmt.parseInt(usize, line[2..], 10) catch continue;
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// Direction of said movement
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const dir: v2d = switch (line[0]) {
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'U' => .{ 0, 1 },
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'D' => .{ 0, -1 },
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'R' => .{ 1, 0 },
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'L' => .{ -1, 0 },
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else => unreachable,
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};
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// Move the head one step at a time
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var step: usize = 0;
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while (step < num) : (step += 1) {
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// Move the head
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knots[0][0] += dir[0];
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knots[0][1] += dir[1];
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// Go through each pair of "head/tail" knots
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var tailIdx: usize = 1;
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while (tailIdx < knots.len) : (tailIdx += 1) {
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const head = &knots[tailIdx - 1];
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const tail = &knots[tailIdx];
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var axis: u8 = 0;
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while (axis < 2) : (axis += 1) {
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const diff = head[axis] - tail[axis];
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const dist = std.math.absInt(diff) catch unreachable;
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if (dist >= 2) {
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// Move it to where the head is -1
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tail[axis] = head[axis] - @divTrunc(dist, diff);
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// If one axis is at a +2 distance, snap the other one
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const otherAxis = (axis + 1) % 2;
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tail[otherAxis] = head[otherAxis];
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break;
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}
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}
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}
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// Register the poisition again if it hasn't been saved before
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const tail = knots[knots.len - 1];
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for (visited.items) |coord| {
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if (coord[0] == tail[0] and coord[1] == tail[1]) break;
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} else {
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visited.append(tail) catch unreachable;
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}
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}
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}
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return visited.items.len;
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}
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