// ConditionExpression support for PutItem, DeleteItem, and UpdateItem
//
// ConditionExpression uses the same grammar as FilterExpression but is evaluated
// against the *existing* item (before the mutation). If the condition evaluates
// to false, the operation is rejected with ConditionalCheckFailedException.
//
// When there is no existing item:
//   - attribute_not_exists(path) → true  (attribute doesn't exist on a non-existent item)
//   - attribute_exists(path)     → false
//   - All comparisons            → false (no attribute to compare)
//
// This file provides:
//   1. parse_condition_expression_string  — extract ConditionExpression from JSON body
//   2. evaluate_condition                 — evaluate parsed condition against an item
//   3. Condition_Result                   — result enum for condition evaluation
package dynamodb

import "core:encoding/json"

// ============================================================================
// Condition Evaluation Result
// ============================================================================

Condition_Result :: enum {
	Passed,              // Condition met (or no condition specified)
	Failed,              // Condition not met → ConditionalCheckFailedException
	Parse_Error,         // Malformed ConditionExpression → ValidationException
}

// ============================================================================
// Request Parsing
// ============================================================================

// Extract the raw ConditionExpression string from the request body.
parse_condition_expression_string :: proc(request_body: []byte) -> (expr: string, ok: bool) {
	data, parse_err := json.parse(request_body, allocator = context.temp_allocator)
	if parse_err != nil {
		return
	}
	defer json.destroy_value(data)

	root, root_ok := data.(json.Object)
	if !root_ok {
		return
	}

	ce_val, found := root["ConditionExpression"]
	if !found {
		return
	}

	ce_str, str_ok := ce_val.(json.String)
	if !str_ok {
		return
	}

	expr = string(ce_str)
	ok = true
	return
}

// ============================================================================
// Full Condition Evaluation Pipeline
//
// Parses ConditionExpression + ExpressionAttributeNames/Values from the
// request body, then evaluates against the existing item.
//
// Parameters:
//   request_body  — full JSON request body
//   existing_item — the item currently in the database (nil if no item exists)
//   attr_names    — pre-parsed ExpressionAttributeNames (caller may already have these)
//   attr_values   — pre-parsed ExpressionAttributeValues
//
// Returns Condition_Result:
//   .Passed      — no ConditionExpression, or condition evaluated to true
//   .Failed      — condition evaluated to false
//   .Parse_Error — ConditionExpression is malformed
// ============================================================================

evaluate_condition_expression :: proc(
	request_body: []byte,
	existing_item: Maybe(Item),
	attr_names: Maybe(map[string]string),
	attr_values: map[string]Attribute_Value,
) -> Condition_Result {
	// Extract ConditionExpression string
	condition_str, has_condition := parse_condition_expression_string(request_body)
	if !has_condition {
		return .Passed // No condition → always pass
	}

	// Parse the condition into a filter tree (same grammar as FilterExpression)
	filter_node, parse_ok := parse_filter_expression(condition_str, attr_names, attr_values)
	if !parse_ok || filter_node == nil {
		return .Parse_Error
	}
	defer {
		filter_node_destroy(filter_node)
	}

	// If there is no existing item, build an empty item for evaluation.
	// This means attribute_not_exists → true, attribute_exists → false,
	// all comparisons → false (attribute not found).
	eval_item: Item
	if item, has_item := existing_item.?; has_item {
		eval_item = item
	} else {
		// Empty item — no attributes exist
		eval_item = Item{}
	}

	if evaluate_filter(eval_item, filter_node) {
		return .Passed
	}

	return .Failed
}