diff --git a/src/day05.zig b/src/day05.zig
index 9828ab6..7913895 100644
--- a/src/day05.zig
+++ b/src/day05.zig
@@ -5,117 +5,179 @@ const mem = std.mem;
 pub fn main() !void {
     const input = @embedFile("data/day05.txt");
     const sln = try solve(util.gpa, input);
-    std.debug.print("{d}\n", .{sln.a});
-    std.debug.print("{d}\n", .{sln.b});
+    std.debug.print("{d}\n", .{sln});
+    std.debug.print("{d}\n", .{sln});
 }
 
-const Solution = struct {
-    a: usize,
-    b: usize,
-};
+fn Range(comptime T: type) type {
+    return struct {
+        start: T,
+        end: T,
 
-const Transform = struct {
-    from: []const u8,
-    to: []const u8,
-};
+        const Self = @This();
+
+        /// Returns the intersection of two ranges.
+        pub fn intersect(self: Self, other: Self) ?Self {
+            const a = if (self.start < other.start) self else other;
+            const b = if (self.start < other.start) other else self;
+            // std.debug.print("{any}/{any}\n", .{ a, b });
+
+            if (a.start == a.end or b.start == b.end) return null; // Points don't intersect.
+
+            if (a.end < b.start) return null; // No intersection.
+            if (a.end > b.end) return b; // A is a superset of B.
+            return .{ .start = b.start, .end = a.end };
+        }
+
+        /// Returns the union of two ranges.
+        pub fn join(self: Self, other: Self) ?Self {
+            if (self.intersect(other) == null) return null; // Can't union disjoint ranges.
+            // std.debug.print("merge {any}+{any}\n", .{ self, other });
+            // Now guaranteed that they overlap.
+            const merged = Self{ .start = @min(self.start, other.start), .end = @max(self.end, other.end) };
+            std.debug.print("= {any}\n", .{merged});
+            return merged;
+        }
+    };
+}
+
+test "range intersect" {
+    const R = Range(usize);
+    const e = .{ .start = 2, .end = 5 };
+    // Superset
+    try std.testing.expectEqual(e, (R{ .start = 0, .end = 6 }).intersect(.{ .start = 2, .end = 5 }));
+    try std.testing.expectEqual(e, (R{ .start = 2, .end = 5 }).intersect(.{ .start = 0, .end = 6 }));
+    // A < B
+    try std.testing.expectEqual(e, (R{ .start = 0, .end = 5 }).intersect(.{ .start = 2, .end = 6 }));
+    // B < A
+    try std.testing.expectEqual(e, (R{ .start = 2, .end = 6 }).intersect(.{ .start = 0, .end = 5 }));
+    // No intersect
+    try std.testing.expect(null == (R{ .start = 0, .end = 3 }).intersect(.{ .start = 4, .end = 6 }));
+}
 
 const Map = struct {
-    dest_start: usize,
-    source_start: usize,
-    range: usize,
+    offset: isize,
+    source: Range(usize),
 };
 
 const Component = struct {
-    name: []const u8,
-    value: usize,
+    stage: usize,
+    range: Range(usize),
 };
 
-const UsizeSet = std.AutoHashMap(usize, void);
+fn mergeRanges(ranges: *std.ArrayList(Component)) void {
+    var i: usize = 0;
+    while (i < ranges.items.len) : (i += 1) {
+        var item = ranges.items[i];
+        var j = i + 1;
+        while (j < ranges.items.len) {
+            const next = ranges.items[j];
+            if (item.range.join(next.range)) |merged| {
+                const merged_item = .{ .stage = item.stage, .range = merged };
+                item = merged_item;
+                ranges.items[i] = merged_item;
+                _ = ranges.swapRemove(j);
+            } else {
+                j += 1;
+            }
+        }
+    }
+}
 
-fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+test "mergeRanges" {
+    var ranges = std.ArrayList(Component).init(std.testing.allocator);
+    defer ranges.deinit();
+    try ranges.append(.{ .stage = 0, .range = .{ .start = 79, .end = 93 } });
+    try ranges.append(.{ .stage = 0, .range = .{ .start = 55, .end = 66 } });
+    try ranges.append(.{ .stage = 0, .range = .{ .start = 70, .end = 10 } });
+    mergeRanges(&ranges);
+    try std.testing.expectEqual(.{ .start = 70, .end = 93 }, ranges.items[0].range);
+    try std.testing.expectEqual(.{ .start = 55, .end = 66 }, ranges.items[1].range);
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !usize {
     var it = util.splitLines(input);
     const seed_line = it.next().?;
     const seeds = try util.parseIntsScalar(usize, alloc, seed_line[mem.indexOfScalar(u8, seed_line, ':').? + 1 ..], .{});
     defer alloc.free(seeds);
 
-    var last_values = UsizeSet.init(alloc);
-    var working_values = UsizeSet.init(alloc);
-    defer last_values.deinit();
-    defer working_values.deinit();
+    var ranges = std.ArrayList(Component).init(alloc);
+    defer ranges.deinit();
+    var lowest_location: usize = std.math.maxInt(usize);
 
-    var low_loc: usize = std.math.maxInt(usize);
-
-    var seeds_index: usize = 0;
-    while (seeds_index < seeds.len) : (seeds_index += 2) {
-        last_values.clearRetainingCapacity();
-        working_values.clearRetainingCapacity();
-
-        std.debug.print("= seeding {d}/{d} =\n", .{ seeds_index, seeds.len });
-        const start = seeds[seeds_index];
-        const range = seeds[seeds_index + 1];
-        for (start..start + range) |seed| {
-            // std.debug.print("seed {d}\n", .{seed});
-            try working_values.put(seed, {});
-        }
-
-        // Reset parse input.
-        it = util.splitLines(input);
-        _ = it.next().?;
-        var current_component: []const u8 = undefined;
-        while (it.next()) |line| {
-            if (line.len == 0) {
-                std.debug.print("== swap ==\n", .{});
-                // Copy all remaining
-                var lv_it = last_values.iterator();
-                while (lv_it.next()) |entry| {
-                    try working_values.put(entry.key_ptr.*, {});
-                }
-                // Swap working sets
-                mem.swap(UsizeSet, &last_values, &working_values);
-                working_values.clearRetainingCapacity();
-            } else if (mem.endsWith(u8, line, "map:")) {
-                var t_it = util.split(line, ' ');
-                var comp_it = util.split(t_it.next().?, '-');
-                _ = comp_it.next(); // A
-                _ = comp_it.next(); // to
-                current_component = comp_it.next().?; // B
-            } else {
-                std.debug.print("== {s} {d}/{d}\n", .{ current_component, working_values.count(), last_values.count() });
-                const map = try util.parseIntsScalar(usize, alloc, line, .{});
-                defer alloc.free(map);
-
-                var to_delete = UsizeSet.init(alloc);
-                defer to_delete.deinit();
-                var lv_it = last_values.iterator();
-                while (lv_it.next()) |entry| {
-                    // if (transitioned[index]) {
-                    //     std.debug.print("{d} already mapped\n", .{index});
-                    //     continue :map_blk;
-                    // }
-                    const item = entry.key_ptr.*;
-                    if (map[1] <= item and item <= map[1] + map[2]) {
-                        const offset = @as(isize, @intCast(map[0])) - @as(isize, @intCast(map[1]));
-                        const res: usize = @intCast(@as(isize, @intCast(item)) + offset);
-                        // std.debug.print("{d} => {d}\n", .{ item, res });
-                        try working_values.put(res, {});
-                        try to_delete.put(item, {});
-                    }
-                }
-                var td_it = to_delete.iterator();
-                while (td_it.next()) |entry| {
-                    // std.debug.print("pop {d}\n", .{entry.key_ptr.*});
-                    _ = last_values.remove(entry.key_ptr.*);
-                }
-            }
-        }
-
-        var lv_it = last_values.iterator();
-        while (lv_it.next()) |value| {
-            low_loc = @min(low_loc, value.key_ptr.*);
+    {
+        std.debug.print("= seeding {d} =\n", .{seeds.len});
+        var index: usize = 0;
+        while (index < seeds.len) : (index += 2) {
+            try ranges.append(.{
+                .stage = 0,
+                .range = .{
+                    .start = seeds[index],
+                    .end = seeds[index] + seeds[index + 1],
+                },
+            });
         }
     }
 
-    return .{ .a = low_loc, .b = 0 };
+    var current_stage: usize = 0;
+    while (it.next()) |line| {
+        if (line.len == 0) {
+            std.debug.print("== finalise ==\n", .{});
+            for (ranges.items, 0..) |item, index| {
+                if (item.stage != current_stage) {
+                    ranges.items[index] = .{ .stage = current_stage, .range = item.range };
+                }
+            }
+            mergeRanges(&ranges);
+        } else if (mem.endsWith(u8, line, "map:")) {
+            current_stage += 1;
+        } else {
+            std.debug.print("== stage {d} map {s} ==\n", .{ current_stage, line });
+            const map_line = try util.parseIntsScalar(usize, alloc, line, .{});
+            defer alloc.free(map_line);
+            const map = Map{
+                .offset = @as(isize, @intCast(map_line[0])) - @as(isize, @intCast(map_line[1])),
+                .source = .{ .start = map_line[1], .end = map_line[1] + map_line[2] },
+            };
+
+            var i: usize = ranges.items.len;
+            while (i != 0) : (i -= 1) {
+                if (ranges.items[i - 1].stage == current_stage) continue;
+                const item = ranges.swapRemove(i - 1);
+                std.debug.print("item {d}/{d} {d}..{d}\n", .{ i, ranges.items.len + 1, item.range.start, item.range.end });
+
+                if (item.range.intersect(.{ .start = 0, .end = map.source.start })) |section| {
+                    // Non-transitioning section.
+                    std.debug.print("=> s {d}..{d}\n", .{ section.start, section.end });
+                    try ranges.append(.{ .stage = item.stage, .range = section });
+                }
+                if (item.range.intersect(map.source)) |section| {
+                    // Transitioning section.
+                    const range = Range(usize){
+                        .start = @intCast(@as(isize, @intCast(section.start)) + map.offset),
+                        .end = @intCast(@as(isize, @intCast(section.end)) + map.offset),
+                    };
+                    std.debug.print("=> m {d}..{d}\n", .{ range.start, range.end });
+                    try ranges.append(.{
+                        .stage = current_stage,
+                        .range = range,
+                    });
+                }
+                if (item.range.intersect(.{ .start = map.source.end, .end = std.math.maxInt(usize) })) |section| {
+                    // Non-transitioning section.
+                    std.debug.print("=> e {d}..{d}\n", .{ section.start, section.end });
+                    try ranges.append(.{ .stage = item.stage, .range = section });
+                }
+            }
+        }
+    }
+
+    for (ranges.items) |item| {
+        std.debug.print("{any}\n", .{item.range});
+        lowest_location = @min(lowest_location, item.range.start);
+    }
+
+    return lowest_location;
 }
 
 test "silver" {
@@ -156,7 +218,7 @@ test "silver" {
     ;
     _ = input;
     // const sln = try solve(std.testing.allocator, input);
-    // try std.testing.expectEqual(@as(usize, 35), sln.a);
+    // try std.testing.expectEqual(@as(usize, 35), sln);
 }
 
 test "gold" {
@@ -196,5 +258,5 @@ test "gold" {
         \\56 93 4
     ;
     const sln = try solve(std.testing.allocator, input);
-    try std.testing.expectEqual(@as(usize, 46), sln.a);
+    try std.testing.expectEqual(@as(usize, 46), sln);
 }