fix: make last(empty) yield no output values

docs/content/manual/dev/manual.yml

@@ -3057,9 +3057,12 @@ sections:
  Note that `nth(n; expr)` doesn't support negative values of `n`.
 
  examples:
- - program: '[first(range(.)), last(range(.)), nth(./2; range(.))]'
+ - program: '[first(range(.)), last(range(.)), nth(5; range(.))]'
  input: '10'
  output: ['[0,9,5]']
+ - program: '[first(empty), last(empty), nth(5; empty)]'
+ input: 'null'
+ output: ['[]']
 
  - title: "`first`, `last`, `nth(n)`"
  body: |

src/builtin.c

@@ -43,10 +43,7 @@ void *alloca (size_t);
 #include "locfile.h"
 #include "jv_unicode.h"
 #include "jv_alloc.h"
-#include "jv_dtoa.h"
-#include "jv_dtoa_tsd.h"
 #include "jv_private.h"
-#include "util.h"
 
 
 #define BINOP(name) \
@@ -1879,6 +1876,32 @@ BINOPS
 #undef LIBM_DD
 #undef LIBM_DA
 
+// This is a hack to make last(empty) yield no output values without using boxing.
+static block gen_last_1() {
+ block last_var = gen_op_var_fresh(STOREV, "last");
+ block is_empty_var = gen_op_var_fresh(STOREV, "is_empty");
+ block init = BLOCK(gen_op_simple(DUP),
+ gen_const(jv_null()),
+ last_var,
+ gen_op_simple(DUP),
+ gen_const(jv_true()),
+ is_empty_var);
+ block call_arg = BLOCK(gen_call("arg", gen_noop()),
+ gen_op_simple(DUP),
+ gen_op_bound(STOREV, last_var),
+ gen_const(jv_false()),
+ gen_op_bound(STOREV, is_empty_var),
+ gen_op_simple(BACKTRACK));
+ block if_empty = gen_op_simple(BACKTRACK);
+ return BLOCK(init,
+ gen_op_target(FORK, call_arg),
+ call_arg,
+ BLOCK(gen_op_bound(LOADVN, is_empty_var),
+ gen_op_target(JUMP_F, if_empty),
+ if_empty,
+ gen_op_bound(LOADVN, last_var)));
+}
+
 struct bytecoded_builtin { const char* name; block code; };
 static block bind_bytecoded_builtins(block b) {
  block builtins = gen_noop();
@@ -1895,6 +1918,7 @@ static block bind_bytecoded_builtins(block b) {
  }
  {
  struct bytecoded_builtin builtin_def_1arg[] = {
+ {"last", gen_last_1()},
  {"path", BLOCK(gen_op_simple(PATH_BEGIN),
  gen_call("arg", gen_noop()),
  gen_op_simple(PATH_END))},

src/builtin.jq

@@ -166,7 +166,6 @@ def all(condition): all(.[]; condition);
 def any(condition): any(.[]; condition);
 def all: all(.[]; .);
 def any: any(.[]; .);
-def last(g): reduce g as $item (null; $item);
 def nth($n; g):
  if $n < 0 then error("nth doesn't support negative indices")
  else label $out | foreach g as $item ($n + 1; . - 1; if . <= 0 then $item, break $out else empty end) end;

tests/jq.test

@@ -338,6 +338,10 @@ null
 10
 [0,9]
 
+[first(range(.)), last(range(.))]
+0
+[]
+
 [nth(0,5,9,10,15; range(.)), try nth(-1; range(.)) catch .]
 10
 [0,5,9,"nth doesn't support negative indices"]