From 69a3e1ee787000ed971da81f7da634147bca0756 Mon Sep 17 00:00:00 2001
From: Felipe Pena <felipensp@gmail.com>
Date: Sun, 12 Oct 2025 13:11:41 -0300
Subject: [PATCH] v: add a fixed array `filter()` method (fix #25469) (#25479)

---
 vlib/v/checker/checker.v               |  3 ++-
 vlib/v/checker/fn.v                    | 10 ++++++++++
 vlib/v/gen/c/array.v                   |  5 +++--
 vlib/v/gen/c/fn.v                      |  3 +++
 vlib/v/tests/array_fixed_filter_test.v | 20 ++++++++++++++++++++
 5 files changed, 38 insertions(+), 3 deletions(-)
 create mode 100644 vlib/v/tests/array_fixed_filter_test.v

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 3b7377fd6..7aff4d171 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -33,7 +33,8 @@ pub const array_builtin_methods = ['filter', 'clone', 'repeat', 'reverse', 'map'
 	'first', 'last', 'pop_left', 'pop', 'delete', 'insert', 'prepend', 'count']
 pub const array_builtin_methods_chk = token.new_keywords_matcher_from_array_trie(array_builtin_methods)
 pub const fixed_array_builtin_methods = ['contains', 'index', 'any', 'all', 'wait', 'map', 'sort',
-	'sorted', 'sort_with_compare', 'sorted_with_compare', 'reverse', 'reverse_in_place', 'count']
+	'sorted', 'sort_with_compare', 'sorted_with_compare', 'reverse', 'reverse_in_place', 'count',
+	'filter']
 pub const fixed_array_builtin_methods_chk = token.new_keywords_matcher_from_array_trie(fixed_array_builtin_methods)
 // TODO: remove `byte` from this list when it is no longer supported
 pub const reserved_type_names = ['bool', 'char', 'i8', 'i16', 'i32', 'int', 'i64', 'u8', 'u16',
diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index d0632dea0..aca23e3ab 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -3665,6 +3665,16 @@ fn (mut c Checker) fixed_array_builtin_method_call(mut node ast.CallExpr, left_t
 		c.expr(mut arg0.expr)
 		c.check_predicate_param(false, elem_typ, node)
 		node.return_type = ast.int_type
+	} else if method_name == 'filter' {
+		if node_args_len != 1 {
+			c.error('`.${method_name}` expected 1 argument, but got ${node_args_len}',
+				node.pos)
+		}
+		// position of `it` doesn't matter
+		scope_register_it(mut node.scope, node.pos, elem_typ)
+		c.expr(mut arg0.expr)
+		c.check_predicate_param(false, elem_typ, node)
+		node.return_type = c.table.find_or_register_array(elem_typ)
 	} else if method_name == 'wait' {
 		elem_sym := c.table.sym(elem_typ)
 		if elem_sym.kind == .thread {
diff --git a/vlib/v/gen/c/array.v b/vlib/v/gen/c/array.v
index 90c5dc7eb..74afc0c97 100644
--- a/vlib/v/gen/c/array.v
+++ b/vlib/v/gen/c/array.v
@@ -973,11 +973,12 @@ fn (mut g Gen) gen_array_filter(node ast.CallExpr) {
 	}
 
 	sym := g.table.final_sym(node.return_type)
-	if sym.kind != .array {
+	if sym.kind !in [.array_fixed, .array] {
 		verror('filter() requires an array')
 	}
 	info := sym.info as ast.Array
 	styp := g.styp(node.return_type)
+	left_sym := g.table.final_sym(node.left_type)
 	elem_type_str := g.styp(info.elem_type)
 	noscan := g.check_noscan(info.elem_type)
 	has_infix_left_var_name := g.write_prepared_tmp_value(past.tmp_var, node, styp, '{0}')
@@ -997,7 +998,7 @@ fn (mut g Gen) gen_array_filter(node ast.CallExpr) {
 	i := g.new_tmp_var()
 	g.writeln('for (${ast.int_type_name} ${i} = 0; ${i} < ${past.tmp_var}_len; ++${i}) {')
 	g.indent++
-	g.write_prepared_var(var_name, info.elem_type, elem_type_str, past.tmp_var, i, true,
+	g.write_prepared_var(var_name, info.elem_type, elem_type_str, past.tmp_var, i, left_sym.kind == .array,
 		false)
 	g.set_current_pos_as_last_stmt_pos()
 	mut is_embed_map_filter := false
diff --git a/vlib/v/gen/c/fn.v b/vlib/v/gen/c/fn.v
index ed3834134..b029d3688 100644
--- a/vlib/v/gen/c/fn.v
+++ b/vlib/v/gen/c/fn.v
@@ -1305,6 +1305,9 @@ fn (mut g Gen) gen_array_method_call(node ast.CallExpr, left_type ast.Type, left
 
 fn (mut g Gen) gen_fixed_array_method_call(node ast.CallExpr, left_type ast.Type) bool {
 	match node.name {
+		'filter' {
+			g.gen_array_filter(node)
+		}
 		'index' {
 			g.gen_array_index(node)
 		}
diff --git a/vlib/v/tests/array_fixed_filter_test.v b/vlib/v/tests/array_fixed_filter_test.v
new file mode 100644
index 000000000..452251cf9
--- /dev/null
+++ b/vlib/v/tests/array_fixed_filter_test.v
@@ -0,0 +1,20 @@
+fn test_filtered_fixed_arrays() {
+	a := [1, 2, 3]!
+	assert a.filter(it > 2).len == 1
+	assert typeof(a.filter(it > 2)).name == '[]int'
+
+	sfa := ['abc', 'def', 'xyz']!
+	sfaf := sfa.filter(it[0] == `d`)
+	assert sfaf == ['def']
+	assert typeof(sfaf).name == '[]string'
+
+	ufa := [u8(99), 77, 61, 120]!
+	ufaf := ufa.filter(it > 80)
+	assert ufaf == [u8(99), 120]
+	assert typeof(ufaf).name == '[]u8'
+
+	fafa := [[1, 2, 3]!, [4, 5, 6]!, [6, 7, 8]!]!
+	fafaf := fafa.filter(it[2] > 5)
+	assert fafaf == [[4, 5, 6]!, [6, 7, 8]!]
+	assert typeof(fafaf).name == '[][3]int'
+}
-- 
2.39.5