Adds jsonCountDepthAndKeys.

Author: atholbro

paseto/src/main/kotlin/net/aholbrook/paseto/protocol/JsonCountDepthAndKeys.kt

@@ -0,0 +1,43 @@
+package net.aholbrook.paseto.protocol
+
+internal fun jsonCountDepthAndKeys(json: String): Pair<Int, Int> {
+    var depth = 0
+    var maxDepth = 0
+    var keys = 0
+    var inString = false
+    var i = 0
+
+    while (i < json.length) {
+        val c = json[i]
+
+        if (inString) {
+            when (c) {
+                '\\' -> ++i
+                '"' -> inString = false
+            }
+        } else {
+            when (c) {
+                '"' -> inString = true
+                '{', '[' -> {
+                    if (++depth > maxDepth) {
+                        maxDepth = depth
+                    }
+                }
+                '}', ']' -> {
+                    if (--depth < 0) {
+                        maxDepth = -1
+                        keys = -1
+                        break
+                    }
+                }
+                ':' -> {
+                    ++keys
+                }
+            }
+        }
+
+        ++i
+    }
+
+    return Pair(maxDepth, keys)
+}

paseto/src/test/kotlin/net/aholbrook/paseto/protocol/JsonCountDepthAndKeysTests.kt

@@ -0,0 +1,66 @@
+package net.aholbrook.paseto.protocol
+
+import io.kotest.matchers.shouldBe
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.params.ParameterizedTest
+import org.junit.jupiter.params.provider.CsvSource
+import org.junit.jupiter.params.provider.ValueSource
+
+class JsonCountDepthAndKeysTests {
+    @ParameterizedTest
+    @CsvSource(
+        value = [
+            "{} | 1",
+            "[] | 1",
+            "{{{}}{}} | 3",
+            "{{}{{}}} | 3",
+            """{"a":1,"b":[1,2,{"c":true}]} | 3""",
+            """{"text":"{[not-structure]}","items":[{"x":"\\\""}]} | 3""",
+        ],
+        delimiterString = " | ",
+    )
+    fun `valid payloads return expected depth`(payload: String, expectedDepth: Int) {
+        jsonCountDepthAndKeys(payload).first shouldBe expectedDepth
+    }
+
+    @ParameterizedTest
+    @ValueSource(strings = [
+        "}",
+        "[]}",
+        "{}]",
+        "}[]",
+        "][[[[[[]]]]]]",
+        "[[[[[[]]]]]]]"
+    ])
+    fun `corrupt json return invalid depth`(json: String) {
+        jsonCountDepthAndKeys(json).first shouldBe -1
+    }
+
+    @Test
+    fun `deeply nested objects are handled`() {
+        val depth = 128
+        val payload = nestedObject(depth)
+
+        jsonCountDepthAndKeys(payload).first shouldBe depth
+    }
+
+    @Test
+    fun `deeply nested arrays are handled`() {
+        val depth = 2000
+        val payload = nestedArray(depth)
+
+        jsonCountDepthAndKeys(payload).first shouldBe depth
+    }
+
+    private fun nestedObject(depth: Int): String {
+        val prefix = "{\"x\":".repeat(depth)
+        val suffix = "}".repeat(depth)
+        return "$prefix 0 $suffix"
+    }
+
+    private fun nestedArray(depth: Int): String {
+        val prefix = "[".repeat(depth)
+        val suffix = "]".repeat(depth)
+        return "$prefix 0 $suffix"
+    }
+}