diff --git a/src/aoc2025/Day09.kt b/src/aoc2025/Day09.kt
new file mode 100644
index 0000000..2763135
--- /dev/null
+++ b/src/aoc2025/Day09.kt
@@ -0,0 +1,133 @@
+package aoc2025
+
+import println
+import readInput
+import splitInts
+import java.util.*
+import kotlin.math.abs
+
+/*
+--- Day 9: Movie Theater ---
+https://adventofcode.com/2025/day/9
+*/
+fun main() {
+
+    val inlineTestInput = """
+7,3
+7,1
+11,1
+11,7
+9,7
+9,5
+2,5
+2,3
+"""
+
+    fun part1(input: List<String>): Long {
+        val coords = input.map { it.splitInts(",") }
+        var maxArea = 0L
+        for (i1 in 0 until coords.size step 2) {
+            for (i2 in i1 + 2 until coords.size step 2) {
+                maxArea = ((abs(coords[i1][0] - coords[i2][0]) + 1).toLong() * (abs(coords[i1][1] - coords[i2][1]) + 1).toLong()).coerceAtLeast(maxArea)
+            }
+        }
+        return maxArea
+    }
+
+    fun findRanges(set: TreeMap<Int, ArrayList<IntRange>>, x1: Int, y1: Int, x2: Int, y2: Int): Boolean {
+        var it = set.ceilingEntry(y1)
+        while (it != null && it.key <= y2) {
+            val xRanges = it.value.filter { it.first() >= x1 && it.last() <= x2 }
+            if (it.key != y1 && it.key != y2 && xRanges.size > 0) return false
+            if ((it.key == y1 || it.key == y2) && xRanges.size > 1) return false
+            // Java TreeMap is crap -- I would like to iterate from first entry to last
+            it = set.ceilingEntry(it.key + 1)
+        }
+        return true
+    }
+
+    fun intersects(coords: List<List<Int>>, x1: Int, y1: Int, x2: Int, y2: Int): Boolean {
+        for (z1 in 0 until coords.size) {
+            val z2 = if (z1 != coords.size - 1) z1 + 1 else 0
+            if (coords[z1][0] == coords[z2][0]) {
+                // vertical
+                val miny = coords[z1][1].coerceAtMost(coords[z2][1])
+                val maxy = coords[z1][1].coerceAtLeast(coords[z2][1])
+                if (coords[z1][0] > x1 && coords[z1][0] < x2 &&
+                    (miny.coerceAtLeast(y1) < maxy.coerceAtMost(y2))
+                )
+                    return true
+            } else {
+                // horizontal
+                val minx = coords[z1][0].coerceAtMost(coords[z2][0])
+                val maxx = coords[z1][0].coerceAtLeast(coords[z2][0])
+                if (coords[z1][1] > y1 && coords[z1][1] < y2 &&
+                    (minx.coerceAtLeast(x1) < maxx.coerceAtMost(x2))
+                )
+                    return true
+            }
+        }
+        return false
+    }
+
+    fun part2(input: List<String>): Long {
+        val coords = input.map { it.splitInts(",") }
+        val xSpans = TreeMap<Int, ArrayList<IntRange>>()
+        val ySpans = TreeMap<Int, ArrayList<IntRange>>()
+        for (i1 in 0 until coords.size) {
+            val i2 = if (i1 != coords.size - 1) i1 + 1 else 0
+            if (i1 and 1 == 1) {
+                xSpans.getOrPut(coords[i1][1]) { ArrayList<IntRange>() }.add(
+                    IntRange(
+                        coords[i1][0].coerceAtMost(coords[i2][0]),
+                        coords[i1][0].coerceAtLeast(coords[i2][0])
+                    )
+                )
+            } else {
+                ySpans.getOrPut(coords[i1][0]) { ArrayList<IntRange>() }.add(
+                    IntRange(
+                        coords[i1][1].coerceAtMost(coords[i2][1]),
+                        coords[i1][1].coerceAtLeast(coords[i2][1])
+                    )
+                )
+            }
+        }
+
+        //xSpans.values.forEach { it.sortBy { it.first } }
+        //ySpans.values.forEach { it.sortBy { it.first } }
+
+        var maxArea = 0L
+        for (i1 in 0 until coords.size step 1) {
+            for (i2 in i1 + 1 until coords.size step 1) {
+                val x1 = coords[i1][0].coerceAtMost(coords[i2][0])
+                val x2 = coords[i1][0].coerceAtLeast(coords[i2][0])
+                val y1 = coords[i1][1].coerceAtMost(coords[i2][1])
+                val y2 = coords[i1][1].coerceAtLeast(coords[i2][1])
+                if (x1 == x2 || y1 == y2) continue
+
+                val area = ((x2 - x1 + 1).toLong() * (y2 - y1 + 1).toLong())
+                if (area > maxArea) {
+//                    if (!findRanges(xSpans, x1, y1, x2, y2) ||
+//                        !findRanges(ySpans, y1, x1, y2, x2)
+//                    ) continue
+                    if (!intersects(coords, x1, y1, x2, y2))
+                        maxArea = area
+                }
+            }
+        }
+        return maxArea
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    //val testInput = readInput("aoc2025/Day09_test")
+    val testInputPart1Result = part1(testInput)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 50L)
+    check(testInputPart2Result == 24L)
+    val input = readInput("aoc2025/Day09")
+    part1(input).println()
+    part2(input).println()
+}