Implement offline evaluation of map

Author: tom-andersen

firebase-firestore/src/main/java/com/google/firebase/firestore/pipeline/evaluation.kt

@@ -1,3 +1,4 @@
+@file:JvmName("Evaluation")
 package com.google.firebase.firestore.pipeline
 
 import com.google.common.math.LongMath
@@ -7,6 +8,7 @@ import com.google.common.math.LongMath.checkedSubtract
 import com.google.firebase.firestore.UserDataReader
 import com.google.firebase.firestore.model.MutableDocument
 import com.google.firebase.firestore.model.Values
+import com.google.firebase.firestore.model.Values.encodeValue
 import com.google.firebase.firestore.model.Values.isNanValue
 import com.google.firebase.firestore.util.Assert
 import com.google.firestore.v1.Value
@@ -393,6 +395,24 @@ internal val evaluateUnixSecondsToTimestamp = unaryFunction { seconds: Long ->
   EvaluateResult.timestamp(seconds, 0)
 }
 
+// === Map Functions ===
+
+internal val evaluateMap: EvaluateFunction = { params ->
+  if (params.size % 2 != 0)
+    throw Assert.fail("Function should have even number of params, but %d were given.", params.size)
+  else block@{ input: MutableDocument ->
+    val map: MutableMap<String, Value> = HashMap(params.size / 2)
+    for (i in params.indices step 2) {
+      val k = params[i](input).value ?: return@block EvaluateResultError
+      if (!k.hasStringValue()) return@block EvaluateResultError
+      val v = params[i + 1](input).value ?: return@block EvaluateResultError
+      // It is against the API contract to include a key more than once.
+      if (map.put(k.stringValue, v) != null) return@block EvaluateResultError
+    }
+    EvaluateResultValue(encodeValue(map))
+  }
+}
+
 // === Helper Functions ===
 
 private inline fun catch(f: () -> EvaluateResult): EvaluateResult =

firebase-firestore/src/main/java/com/google/firebase/firestore/pipeline/expressions.kt

@@ -1786,7 +1786,7 @@ abstract class Expr internal constructor() {
       FunctionExpr("str_concat", evaluateStrConcat, fieldName, *otherStrings)
 
     internal fun map(elements: Array<out Expr>): Expr =
-      FunctionExpr("map", notImplemented, elements)
+      FunctionExpr("map", evaluateMap, elements)
 
     /**
      * Creates an expression that creates a Firestore map value from an input object.