all: support slices with negative indexes `#[start..end]` (gated arrays) (#12914)

vlib/builtin/array.v

@@ -384,6 +384,57 @@ fn (a array) slice(start int, _end int) array {
 	return res
 }
 
+// slice_ni returns an array using the same buffer as original array
+// but starting from the `start` element and ending with the element before
+// the `end` element of the original array.
+// This function can use negative indexes `a.slice_ni(-3, a.len)`
+// that get the last 3 elements of the array otherwise it return an empty array.
+// This function always return a valid array.
+fn (a array) slice_ni(_start int, _end int) array {
+	mut end := _end
+	mut start := _start
+
+	if start < 0 {
+		start = a.len + start
+		if start < 0 {
+			start = 0
+		}
+	}
+
+	if end < 0 {
+		end = a.len + end
+		if end < 0 {
+			end = 0
+		}
+	}
+	if end >= a.len {
+		end = a.len
+	}
+
+	if start >= a.len || start > end {
+		res := array{
+			element_size: a.element_size
+			data: a.data
+			offset: 0
+			len: 0
+			cap: 0
+		}
+		return res
+	}
+
+	offset := start * a.element_size
+	data := unsafe { &byte(a.data) + offset }
+	l := end - start
+	res := array{
+		element_size: a.element_size
+		data: data
+		offset: a.offset + offset
+		len: l
+		cap: l
+	}
+	return res
+}
+
 // used internally for [2..4]
 fn (a array) slice2(start int, _end int, end_max bool) array {
 	end := if end_max { a.len } else { _end }

vlib/builtin/gated_array_string_test.v

@@ -0,0 +1,78 @@
+fn test_gated_arrays() {
+	a := [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+	assert a#[-1..] == [9]
+	assert a#[..-9] == [0]
+	assert a#[-9..-7] == [1, 2]
+	assert a#[-2..] == [8, 9]
+
+	// fixed array
+	a1 := [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]!
+	assert a1#[-1..] == [9]
+	assert a1#[..-9] == [0]
+	assert a1#[-9..-7] == [1, 2]
+	assert a1#[-2..] == [8, 9]
+
+	// empty array
+	assert a#[-3..-4] == [] // start > end
+	assert a#[20..] == [] // start > array.len
+	assert a#[-20..-10] == [] // start+len < 0
+	assert a#[20..-9] == [] // start > end  && start > end
+}
+
+fn test_gated_strings() {
+	a := '0123456789'
+	assert a#[-1..] == '9'
+	assert a#[..-9] == '0'
+	assert a#[-9..-7] == '12'
+	assert a#[-2..] == '89'
+
+	// empty string
+	assert a#[-3..-4] == '' // start > end
+	assert a#[20..] == '' // start > array.len
+	assert a#[-20..-10] == '' // start+len < 0
+	assert a#[20..-9] == '' // start > end  && start > end
+
+	//
+	// test negative indexes in slices from github discussion
+	//
+	s := '0123456789'
+
+	// normal behaviour
+	assert s#[1..3] == '12'
+	assert s#[..3] == '012'
+	assert s#[8..] == '89'
+
+	// negative indexes behaviour
+	assert s#[-2..] == '89'
+	assert s#[..-8] == '01'
+	assert s#[2..-2] == '234567'
+	assert s#[-12..-16] == ''
+	assert s#[-8..-2] == '234567'
+
+	// out of bound both indexes
+	assert s#[12..14] == ''
+	assert s#[-12..16] == '0123456789'
+}
+
+fn test_gated_mixed_strings() {
+	//
+	// test negative indexes in slices
+	//
+	a := [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+
+	// normal behaviour
+	assert a#[1..3].str() == '[1, 2]'
+	assert a#[..3].str() == '[0, 1, 2]'
+	assert a#[8..].str() == '[8, 9]'
+
+	// negative indexes behaviour
+	assert a#[-2..].str() == '[8, 9]'
+	assert a#[..-8].str() == '[0, 1]'
+	assert a#[2..-2].str() == '[2, 3, 4, 5, 6, 7]'
+	assert a#[-12..-16].str() == '[]'
+	assert a#[-8..-2].str() == '[2, 3, 4, 5, 6, 7]'
+
+	// out of bound both indexes
+	assert a#[12..14].str() == '[]'
+	assert a#[-12..16].str() == a.str()
+}

vlib/builtin/string.v

@@ -788,6 +788,60 @@ pub fn (s string) substr(start int, end int) string {
 	return res
 }
 
+// substr_ni returns the string between index positions `start` and `end` allowing negative indexes
+// This function always return a valid string.
+[direct_array_access]
+pub fn (s string) substr_ni(_start int, _end int) string {
+	mut start := _start
+	mut end := _end
+
+	// borders math
+	if start < 0 {
+		start = s.len + start
+		if start < 0 {
+			start = 0
+		}
+	}
+
+	if end < 0 {
+		end = s.len + end
+		if end < 0 {
+			end = 0
+		}
+	}
+	if end >= s.len {
+		end = s.len
+	}
+
+	if start > s.len || end < start {
+		mut res := string{
+			str: unsafe { malloc_noscan(1) }
+			len: 0
+		}
+		unsafe {
+			res.str[0] = 0
+		}
+		return res
+	}
+
+	len := end - start
+
+	// string copy
+	mut res := string{
+		str: unsafe { malloc_noscan(len + 1) }
+		len: len
+	}
+	for i in 0 .. len {
+		unsafe {
+			res.str[i] = s.str[start + i]
+		}
+	}
+	unsafe {
+		res.str[len] = 0
+	}
+	return res
+}
+
 // index returns the position of the first character of the input string.
 // It will return `-1` if the input string can't be found.
 [direct_array_access]

vlib/v/ast/ast.v

@@ -822,6 +822,7 @@ pub mut:
 	is_farray bool
 	is_option bool // IfGuard
 	is_direct bool // Set if the underlying memory can be safely accessed
+	is_gated  bool // #[] gated array
 }
 
 pub struct IfExpr {
@@ -1207,6 +1208,7 @@ pub:
 	has_high bool
 	has_low  bool
 	pos      token.Position
+	is_gated bool // #[] gated array
 }
 
 pub struct CastExpr {

vlib/v/checker/checker.v

@@ -4331,7 +4331,7 @@ pub fn (mut c Checker) prefix_expr(mut node ast.PrefixExpr) ast.Type {
 	return right_type
 }
 
-fn (mut c Checker) check_index(typ_sym &ast.TypeSymbol, index ast.Expr, index_type ast.Type, pos token.Position, range_index bool) {
+fn (mut c Checker) check_index(typ_sym &ast.TypeSymbol, index ast.Expr, index_type ast.Type, pos token.Position, range_index bool, is_gated bool) {
 	index_type_sym := c.table.sym(index_type)
 	// println('index expr left=$typ_sym.name $node.pos.line_nr')
 	// if typ_sym.kind == .array && (!(ast.type_idx(index_type) in ast.number_type_idxs) &&
@@ -4345,7 +4345,7 @@ fn (mut c Checker) check_index(typ_sym &ast.TypeSymbol, index ast.Expr, index_ty
 			}
 			c.error('$type_str', pos)
 		}
-		if index is ast.IntegerLiteral {
+		if index is ast.IntegerLiteral && !is_gated {
 			if index.val[0] == `-` {
 				c.error('negative index `$index.val`', index.pos)
 			} else if typ_sym.kind == .array_fixed {
@@ -4428,11 +4428,11 @@ pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 	if mut node.index is ast.RangeExpr { // [1..2]
 		if node.index.has_low {
 			index_type := c.expr(node.index.low)
-			c.check_index(typ_sym, node.index.low, index_type, node.pos, true)
+			c.check_index(typ_sym, node.index.low, index_type, node.pos, true, node.is_gated)
 		}
 		if node.index.has_high {
 			index_type := c.expr(node.index.high)
-			c.check_index(typ_sym, node.index.high, index_type, node.pos, true)
+			c.check_index(typ_sym, node.index.high, index_type, node.pos, true, node.is_gated)
 		}
 		// array[1..2] => array
 		// fixed_array[1..2] => array
@@ -4460,7 +4460,11 @@ pub fn (mut c Checker) index_expr(mut node ast.IndexExpr) ast.Type {
 			}
 		} else {
 			index_type := c.expr(node.index)
-			c.check_index(typ_sym, node.index, index_type, node.pos, false)
+			// for [1] case #[1] is not allowed!
+			if node.is_gated == true {
+				c.error('`#[]` allowed only for ranges', node.pos)
+			}
+			c.check_index(typ_sym, node.index, index_type, node.pos, false, false)
 		}
 		value_type := c.table.value_type(typ)
 		if value_type != ast.void_type {

vlib/v/fmt/fmt.v

@@ -1898,6 +1898,11 @@ pub fn (mut f Fmt) if_guard_expr(node ast.IfGuardExpr) {
 
 pub fn (mut f Fmt) index_expr(node ast.IndexExpr) {
 	f.expr(node.left)
+	if node.index is ast.RangeExpr {
+		if node.index.is_gated {
+			f.write('#')
+		}
+	}
 	f.write('[')
 	f.expr(node.index)
 	f.write(']')

vlib/v/fmt/tests/gated_array_keep.vv

@@ -0,0 +1,5 @@
+a := [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+assert a#[-1..] == [9]
+assert a#[..-9] == [0]
+assert a#[-9..-7] == [1, 2]
+assert a#[-2..] == [8, 9]

vlib/v/gen/c/index.v

@@ -60,10 +60,18 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 fn (mut g Gen) range_expr(node ast.IndexExpr, range ast.RangeExpr) {
 	sym := g.table.final_sym(node.left_type)
 	if sym.kind == .string {
-		g.write('string_substr(')
+		if node.is_gated {
+			g.write('string_substr_ni(')
+		} else {
+			g.write('string_substr(')
+		}
 		g.expr(node.left)
 	} else if sym.kind == .array {
-		g.write('array_slice(')
+		if node.is_gated {
+			g.write('array_slice_ni(')
+		} else {
+			g.write('array_slice(')
+		}
 		if node.left_type.is_ptr() {
 			g.write('*')
 		}
@@ -72,7 +80,12 @@ fn (mut g Gen) range_expr(node ast.IndexExpr, range ast.RangeExpr) {
 		// Convert a fixed array to V array when doing `fixed_arr[start..end]`
 		info := sym.info as ast.ArrayFixed
 		noscan := g.check_noscan(info.elem_type)
-		g.write('array_slice(new_array_from_c_array${noscan}(')
+		if node.is_gated {
+			g.write('array_slice_ni(')
+		} else {
+			g.write('array_slice(')
+		}
+		g.write('new_array_from_c_array${noscan}(')
 		g.write('$info.size')
 		g.write(', $info.size')
 		g.write(', sizeof(')

vlib/v/parser/expr.v

@@ -361,8 +361,15 @@ pub fn (mut p Parser) expr_with_left(left ast.Expr, precedence int, is_stmt_iden
 				return node
 			}
 			p.is_stmt_ident = is_stmt_ident
-		} else if p.tok.kind == .lsbr && (p.inside_fn || p.tok.line_nr == p.prev_tok.line_nr) {
+		} else if p.tok.kind in [.lsbr, .nilsbr]
-			node = p.index_expr(node)
+			&& (p.inside_fn || p.tok.line_nr == p.prev_tok.line_nr) {
+			// node = p.index_expr(node)
+			if p.tok.kind == .nilsbr {
+				node = p.index_expr(node, true)
+			} else {
+				node = p.index_expr(node, false)
+			}
+
 			p.is_stmt_ident = is_stmt_ident
 			if p.tok.kind == .lpar && p.tok.line_nr == p.prev_tok.line_nr && node is ast.IndexExpr {
 				p.next()

vlib/v/parser/parse_type.v

@@ -8,8 +8,8 @@ import v.ast
 import v.util
 import v.token
 
-pub fn (mut p Parser) parse_array_type() ast.Type {
+pub fn (mut p Parser) parse_array_type(expecting token.Kind) ast.Type {
-	p.check(.lsbr)
+	p.check(expecting)
 	// fixed array
 	if p.tok.kind in [.number, .name] {
 		mut fixed_size := 0
@@ -88,7 +88,7 @@ pub fn (mut p Parser) parse_array_type() ast.Type {
 	mut nr_dims := 1
 	// detect attr
 	not_attr := p.peek_tok.kind != .name && p.peek_token(2).kind !in [.semicolon, .rsbr]
-	for p.tok.kind == .lsbr && not_attr {
+	for p.tok.kind == expecting && not_attr {
 		p.next()
 		p.check(.rsbr)
 		nr_dims++
@@ -408,9 +408,9 @@ pub fn (mut p Parser) parse_any_type(language ast.Language, is_ptr bool, check_d
 			// func
 			return p.parse_fn_type('')
 		}
-		.lsbr {
+		.lsbr, .nilsbr {
 			// array
-			return p.parse_array_type()
+			return p.parse_array_type(p.tok.kind)
 		}
 		.lpar {
 			// multiple return

vlib/v/parser/parser.v

@@ -2354,7 +2354,7 @@ pub fn (mut p Parser) name_expr() ast.Expr {
 	return node
 }
 
-fn (mut p Parser) index_expr(left ast.Expr) ast.IndexExpr {
+fn (mut p Parser) index_expr(left ast.Expr, is_gated bool) ast.IndexExpr {
 	// left == `a` in `a[0]`
 	start_pos := p.tok.position()
 	p.next() // [
@@ -2379,7 +2379,9 @@ fn (mut p Parser) index_expr(left ast.Expr) ast.IndexExpr {
 				high: high
 				has_high: has_high
 				pos: pos
+				is_gated: is_gated
 			}
+			is_gated: is_gated
 		}
 	}
 	expr := p.expr(0) // `[expr]` or  `[expr..`
@@ -2403,7 +2405,9 @@ fn (mut p Parser) index_expr(left ast.Expr) ast.IndexExpr {
 				has_high: has_high
 				has_low: has_low
 				pos: pos
+				is_gated: is_gated
 			}
+			is_gated: is_gated
 		}
 	}
 	// [expr]
@@ -2433,6 +2437,7 @@ fn (mut p Parser) index_expr(left ast.Expr) ast.IndexExpr {
 					stmts: or_stmts
 					pos: or_pos
 				}
+				is_gated: is_gated
 			}
 		}
 		// `a[i] ?`
@@ -2451,6 +2456,7 @@ fn (mut p Parser) index_expr(left ast.Expr) ast.IndexExpr {
 			stmts: or_stmts
 			pos: or_pos
 		}
+		is_gated: is_gated
 	}
 }
 

vlib/v/scanner/scanner.v

@@ -913,6 +913,12 @@ fn (mut s Scanner) text_scan() token.Token {
 				return s.new_token(.dot, '', 1)
 			}
 			`#` {
+				// manage gated arrays/strings
+				if nextc == `[` {
+					s.pos++
+					return s.new_token(.nilsbr, '', 2)
+				}
+
 				start := s.pos + 1
 				s.ignore_line()
 				if nextc == `!` {

vlib/v/token/token.v

@@ -69,6 +69,7 @@ pub enum Kind {
 	lpar // (
 	rpar // )
 	lsbr // [
+	nilsbr // #[
 	rsbr // ]
 	eq // ==
 	ne // !=
@@ -243,6 +244,7 @@ fn build_token_str() []string {
 	s[Kind.lpar] = '('
 	s[Kind.rpar] = ')'
 	s[Kind.lsbr] = '['
+	s[Kind.nilsbr] = '#['
 	s[Kind.rsbr] = ']'
 	s[Kind.eq] = '=='
 	s[Kind.ne] = '!='
@@ -379,6 +381,7 @@ pub enum Precedence {
 pub fn build_precedences() []Precedence {
 	mut p := []Precedence{len: int(Kind._end_)}
 	p[Kind.lsbr] = .index
+	p[Kind.nilsbr] = .index
 	p[Kind.dot] = .call
 	// `++` | `--` | `?`
 	p[Kind.inc] = .postfix