1 files changed, 68 insertions, 0 deletions
diff --git a/day6.zig b/day6.zig
new file mode 100644
index 0000000..94023a0
--- /dev/null
+++ b/day6.zig
@@ -0,0 +1,68 @@
+const std = @import("std");
+
+pub fn main() !void {
+    try std.fmt.format(std.io.getStdOut().writer(), "day6 pt1: {}\n", .{try solve_pt1(std.heap.page_allocator, puzzle_input)});
+    try std.fmt.format(std.io.getStdOut().writer(), "day6 pt2: {}\n", .{try solve_pt1(std.heap.page_allocator, puzzle_input2)});
+}
+
+fn readnums(a: std.mem.Allocator, input: []const u8) ![]u64 {
+    var toks = std.mem.tokenize(u8, input, " ");
+    _  = toks.next() orelse return error.NoLabel;
+    var al = std.ArrayList(u64).init(a);
+    defer al.deinit();
+    while (toks.next()) |tok| {
+        try al.append(try std.fmt.parseInt(u64, tok, 10));
+    }
+    return try al.toOwnedSlice();
+}
+
+fn solve_pt1(a: std.mem.Allocator, input: []const u8) !u64 {
+    var spl = std.mem.split(u8, input, "\n");
+    const times = try readnums(a, spl.first());
+    defer a.free(times);
+    const distances = try readnums(a, spl.next() orelse return error.NoDistance);
+    defer a.free(distances);
+
+    var sum: ?u64 = null;
+    for (times, distances) |time, distance| {
+        const ftime: f64 = @floatFromInt(time);
+        const fdistance: f64 = @floatFromInt(distance);
+        const disc = (std.math.pow(f64, ftime, 2)) - 4 * fdistance;
+        if (disc <= 0) {
+            // std.log.warn("discriminant {}, no possible solutions", .{disc});
+            continue;
+        }
+        const x_low: f64 = ((ftime) - @sqrt(disc)) / 2 ;
+        const x_high: f64 = ((ftime) + @sqrt(disc)) / 2 ;
+        // std.log.warn("x_low {} x_high {}", .{x_low, x_high});
+        // So these are _strict_ bounds. I want the next integer _strictly_ greater than x_low
+        // and the next integer _strictly_ less than x_high
+        const x_low_i: u64 = if (@ceil(x_low) == x_low) (@as(u64, @intFromFloat(x_low)) + 1) else @intFromFloat(@ceil(x_low));
+        const x_high_i: u64 = if (@floor(x_high) == x_high) (@as(u64, @intFromFloat(x_high)) - 1) else @intFromFloat(@floor(x_high));
+        const diff = (x_high_i + 1) - x_low_i; // inclusive range
+        
+        if (sum) |s| {
+            sum = s * diff;
+        } else {
+            sum = diff;
+        }
+    }
+    return sum orelse error.NoRaces;
+}
+
+test "pt1" {
+    try std.testing.expectEqual(@as(u64, 288), try solve_pt1(std.testing.allocator, test_input));
+}
+
+const test_input = 
+\\Time:      7  15   30
+\\Distance:  9  40  200
+;
+const puzzle_input = 
+\\Time:        49     97     94     94
+\\Distance:   263   1532   1378   1851
+;
+const puzzle_input2 = 
+\\Time:        49979494
+\\Distance:   263153213781851
+;
+\ No newline at end of file