From 2cac846a4d898c6c76a970735756301d9dc79d0d Mon Sep 17 00:00:00 2001
From: "M.V. Hutz" <git@maximhutz.me>
Date: Fri, 13 Feb 2026 18:29:01 -0500
Subject: [PATCH] feat: friday

---
 pkg/pacific_atlantic_water_flow/main.go      | 105 +++++++++++++++++++
 pkg/pacific_atlantic_water_flow/main_test.go |  62 +++++++++++
 2 files changed, 167 insertions(+)
 create mode 100644 pkg/pacific_atlantic_water_flow/main.go
 create mode 100644 pkg/pacific_atlantic_water_flow/main_test.go

diff --git a/pkg/pacific_atlantic_water_flow/main.go b/pkg/pacific_atlantic_water_flow/main.go
new file mode 100644
index 0000000..8d96d64
--- /dev/null
+++ b/pkg/pacific_atlantic_water_flow/main.go
@@ -0,0 +1,105 @@
+package pacificatlanticwaterflow
+
+type Position struct{ X, Y int }
+
+func (p Position) Neighbors() []Position {
+	return []Position{
+		{p.X + 1, p.Y},
+		{p.X - 1, p.Y},
+		{p.X, p.Y + 1},
+		{p.X, p.Y - 1},
+	}
+}
+
+func (p Position) ToList() []int {
+	return []int{p.Y, p.X}
+}
+
+type Board [][]int
+
+func (b Board) Get(p Position) int {
+	if b.OutOfBounds(p) {
+		return -1
+	}
+
+	return b[p.Y][p.X]
+}
+
+func (b Board) Height() int { return len(b) }
+func (b Board) Width() int  { return len(b[0]) }
+
+func (b Board) OutOfBounds(p Position) bool {
+	return p.Y < 0 || p.Y >= b.Height() || p.X < 0 || p.X >= b.Width()
+}
+
+type Flow struct{ Pacific, Atlantic bool }
+
+func (f *Flow) Merge(o Flow) {
+	f.Atlantic = f.Atlantic || o.Atlantic
+	f.Pacific = f.Pacific || o.Pacific
+}
+
+func (f Flow) Both() bool {
+	return f.Pacific && f.Atlantic
+}
+
+func (b Board) GetFlow(p Position) Flow {
+	return Flow{
+		Atlantic: p.Y >= b.Height() || p.X >= b.Width(),
+		Pacific:  p.Y < 0 || p.X < 0,
+	}
+}
+
+func pacificAtlanticHelper(board Board, position Position, flows map[Position]Flow) Flow {
+	if flow, ok := flows[position]; ok {
+		return flow
+	}
+
+	if board.OutOfBounds(position) {
+		return board.GetFlow(position)
+	}
+
+	flow := Flow{}
+
+	var top Position
+	plateau := map[Position]bool{}
+	unvisited := []Position{position}
+	for len(unvisited) > 0 {
+		top, unvisited = unvisited[len(unvisited)-1], unvisited[:len(unvisited)-1]
+
+		if board.Get(top) < board.Get(position) {
+			flow.Merge(pacificAtlanticHelper(board, top, flows))
+		} else if board.Get(top) == board.Get(position) {
+			if _, ok := plateau[top]; !ok {
+				plateau[top] = true
+				unvisited = append(unvisited, top.Neighbors()...)
+			}
+		}
+	}
+
+	for mem := range plateau {
+		flows[mem] = flow
+	}
+
+	return flow
+}
+
+func PacificAtlantic(heights [][]int) [][]int {
+	flows := map[Position]Flow{}
+	board := Board(heights)
+
+	for y := range board.Height() {
+		for x := range board.Width() {
+			pacificAtlanticHelper(board, Position{x, y}, flows)
+		}
+	}
+
+	result := [][]int{}
+	for position, flow := range flows {
+		if flow.Both() {
+			result = append(result, position.ToList())
+		}
+	}
+
+	return result
+}
diff --git a/pkg/pacific_atlantic_water_flow/main_test.go b/pkg/pacific_atlantic_water_flow/main_test.go
new file mode 100644
index 0000000..fa1d180
--- /dev/null
+++ b/pkg/pacific_atlantic_water_flow/main_test.go
@@ -0,0 +1,62 @@
+package pacificatlanticwaterflow
+
+import (
+	"sort"
+	"testing"
+)
+
+func sortResult(result [][]int) {
+	sort.Slice(result, func(i, j int) bool {
+		if result[i][0] != result[j][0] {
+			return result[i][0] < result[j][0]
+		}
+		return result[i][1] < result[j][1]
+	})
+}
+
+func TestPacificAtlantic(t *testing.T) {
+	tests := []struct {
+		name     string
+		heights  [][]int
+		expected [][]int
+	}{
+		{
+			name: "5x5 grid",
+			heights: [][]int{
+				{1, 2, 2, 3, 5},
+				{3, 2, 3, 4, 4},
+				{2, 4, 5, 3, 1},
+				{6, 7, 1, 4, 5},
+				{5, 1, 1, 2, 4},
+			},
+			expected: [][]int{{0, 4}, {1, 3}, {1, 4}, {2, 2}, {3, 0}, {3, 1}, {4, 0}},
+		},
+		{
+			name: "3x2 all ones",
+			heights: [][]int{
+				{1, 1},
+				{1, 1},
+				{1, 1},
+			},
+			expected: [][]int{{0, 0}, {0, 1}, {1, 0}, {1, 1}, {2, 0}, {2, 1}},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			result := PacificAtlantic(tc.heights)
+			sortResult(result)
+			sortResult(tc.expected)
+
+			if len(result) != len(tc.expected) {
+				t.Fatalf("expected %d results, got %d: %v", len(tc.expected), len(result), result)
+			}
+
+			for i := range tc.expected {
+				if result[i][0] != tc.expected[i][0] || result[i][1] != tc.expected[i][1] {
+					t.Errorf("result[%d] = %v, expected %v", i, result[i], tc.expected[i])
+				}
+			}
+		})
+	}
+}