io: add a string_reader submodule (#20893)

vlib/strings/builder.c.v

@@ -154,8 +154,9 @@ pub fn (mut b Builder) go_back(n int) {
 	b.trim(b.len - n)
 }
 
+// spart returns a part of the buffer as a string
 @[inline]
-fn (b &Builder) spart(start_pos int, n int) string {
+pub fn (b &Builder) spart(start_pos int, n int) string {
 	unsafe {
 		mut x := malloc_noscan(n + 1)
 		vmemcpy(x, &u8(b.data) + start_pos, n)
@@ -253,6 +254,20 @@ pub fn (mut b Builder) ensure_cap(n int) {
 	}
 }
 
+// grow_len grows the length of the buffer by `n` bytes if necessary
+@[unsafe]
+pub fn (mut b Builder) grow_len(n int) {
+	if n <= 0 {
+		return
+	}
+
+	new_len := b.len + n
+	b.ensure_cap(new_len)
+	unsafe {
+		b.len = new_len
+	}
+}
+
 // free frees the memory block, used for the buffer.
 // Note: do not use the builder, after a call to free().
 @[unsafe]

vlib/strings/builder_test.v

@@ -166,3 +166,28 @@ fn test_write_decimal() {
 	assert sb_i64_str(9223372036854775807) == '9223372036854775807'
 	assert sb_i64_str(-9223372036854775807) == '-9223372036854775807'
 }
+
+fn test_grow_len() {
+	mut sb := strings.new_builder(10)
+	assert sb.len == 0
+	assert sb.cap == 10
+
+	sb.write_string('0123456789')
+	assert sb.len == 10
+
+	unsafe { sb.grow_len(-5) }
+	assert sb.len == 10
+	assert sb.cap == 10
+
+	unsafe { sb.grow_len(10) }
+	assert sb.len == 20
+	assert sb.cap == 20
+
+	unsafe { sb.ensure_cap(35) }
+	assert sb.len == 20
+	assert sb.cap == 35
+
+	unsafe { sb.grow_len(5) }
+	assert sb.len == 25
+	assert sb.cap == 35
+}