1 files changed, 190 insertions, 0 deletions
diff --git a/day10.zig b/day10.zig
new file mode 100644
index 0000000..3d52a1f
--- /dev/null
+++ b/day10.zig
@@ -0,0 +1,190 @@
+const std = @import("std");
+
+pub fn main() !void {
+    try std.fmt.format(std.io.getStdOut().writer(), "day 10 pt1: {}\n", .{try solve_pt1(std.heap.page_allocator, puzzle_input)});
+}
+
+const Direction = enum { north, east, south, west, 
+    fn opp(self: Direction) Direction {
+        return switch (self) {
+            .north => .south,
+            .east => .west,
+            .south => .north,
+            .west => .east,
+        };
+    }
+};
+
+const Cell = enum(u8) {
+    NS = '|',
+    WE = '-',
+    NE = 'L',
+    NW = 'J',
+    SW = '7',
+    SE = 'F',
+    Ground = '.',
+    Start = 'S',
+    _,
+
+    fn next(self: Cell, whence: Direction) ?Direction {
+        // return switch (self) {
+        //     .NS => switch (whence) {
+        //         .north => .South,
+        //         .south => .North,
+        //         else => null,
+        //     },
+        //     else => null,
+        // };
+        // std.log.warn("whence {} cell {}", .{whence, self});
+        return switch (whence) {
+            .north => switch (self) {
+                .NS => .south,
+                .NE => .east,
+                .NW => .west,
+                else => null,
+            },
+            .east => switch (self) {
+                .WE => .west,
+                .NE => .north,
+                .SE => .south,
+                else => null,
+            },
+            .south => switch (self) {
+                .NS => .north,
+                .SW => .west,
+                .SE => .east,
+                else => null,
+            },
+            .west => switch (self) {
+                .WE => .east,
+                .NW => .north,
+                .SW => .south,
+                else => null,
+            },
+        };
+    }
+};
+
+const Coord = struct {
+    x: i64,
+    y: i64,
+
+    fn move(self: Coord, dir: Direction) Coord {
+        return switch (dir) {
+            .north => .{ .x = self.x, .y = self.y - 1 },
+            .south => .{ .x = self.x, .y = self.y + 1 },
+            .east => .{ .x = self.x + 1, .y = self.y },
+            .west => .{ .x = self.x - 1, .y = self.y },
+        };
+    }
+};
+
+const Path = struct {
+    grid: *const Grid,
+    current: Coord,
+    // Doesn't include .current
+    trail: std.ArrayList(Coord),
+    // direction we travelled to get to .current
+    whence: Direction,
+};
+
+const Grid = struct {
+    data: []const u8,
+    width: usize,
+    height: usize,
+
+    fn parse(input: []const u8) !Grid {
+        return .{
+            .data = input,
+            .width = std.mem.indexOfScalar(u8, input, '\n') orelse return error.Foo,
+            .height = std.mem.count(u8, input, "\n") + 1,
+        };
+    }
+
+    fn get(self: Grid, c: Coord) ?Cell {
+        if (c.x < 0) return null;
+        if (c.y < 0) return null;
+        if (c.x >= self.width) return null;
+        if (c.y >= self.height) return null;
+        return @enumFromInt(self.data[@intCast(c.x + (c.y * (@as(i64, @intCast(self.width)) + 1)))]); // skip the newline
+    }
+
+    fn findStart(self: Grid) ?Coord {
+        for (0..self.width) |x| {
+            for (0..self.height) |y| {
+                const coord: Coord = .{ .x = @intCast(x), .y = @intCast(y) };
+                //std.log.warn("checking {any} for start", .{coord});
+                if (self.get(coord) == .Start) {
+                    return coord;
+                }
+            }
+        }
+        return null;
+    }
+};
+
+fn solve_pt1(a: std.mem.Allocator, input: []const u8) !u64 {
+    // read the Grid
+    // find the start pos
+    // setup 4 Paths on each NESW from S and follow them through, one of them
+    // will link up back to S.
+    // half the trail length gives the answer.
+    const grid = try Grid.parse(input);
+    // std.log.warn("grid width {} height {}", .{ grid.width, grid.height });
+
+    const start: Coord = grid.findStart() orelse return error.NoStart;
+
+    const loop: Path = lp1: for (std.enums.values(Direction)) |dir| {
+        const st = start.move(dir);
+        // std.log.warn("st {any}", .{st});
+        _ = grid.get(st) orelse continue :lp1;
+        var trail = std.ArrayList(Coord).init(a);
+        defer trail.deinit();
+        try trail.append(start);
+        var path: Path = .{
+            .current = st,
+            .grid = &grid,
+            .trail = trail,
+            .whence = dir.opp(),
+        };
+        while (true) {
+            const cell: Cell = grid.get(path.current) orelse {
+                // std.log.warn("out of bounds", .{});
+                continue :lp1; 
+            }; // out of bounds
+            if (cell == .Start) break :lp1 Path{
+                .current = path.current,
+                .grid = path.grid,
+                .trail = try trail.clone(),
+                .whence = path.whence,
+            }; // we found it yay
+            const dirn = cell.next(path.whence) orelse {
+                // std.log.warn("pipe mismatch", .{});
+                continue :lp1;
+            }; // pipes don't match up
+            const newpos = path.current.move(dirn);
+            path.whence = dirn.opp();
+            try trail.append(path.current);
+            path.current = newpos;
+        }
+    } else {
+        return error.NoLoop;
+    };
+    defer loop.trail.deinit();
+    // std.log.warn("loop {any}", .{loop});
+    return loop.trail.items.len / 2;
+}
+
+test "pt1_1" {
+    try std.testing.expectEqual(@as(u64, 4), try solve_pt1(std.testing.allocator, test_input1));
+}
+
+const test_input1 =
+    \\.....
+    \\.S-7.
+    \\.|.|.
+    \\.L-J.
+    \\.....
+;
+
+const puzzle_input = @embedFile("day10.in");