add skip/2 as the counterpart to limit/2 (#3181)

I also changed the behavior of limit/2 with negative count to emit an error, like nth/2 does.
Also, I redefined nth/2 using skip/2 to minimize the impact of growth of builtin filters.

docs/content/manual/dev/manual.yml

@@ -3037,16 +3037,26 @@ sections:
             input: '[1,2,3]'
             output: ['false']
 
-      - title: "`limit(n; exp)`"
+      - title: "`limit(n; expr)`"
         body: |
 
-          The `limit` function extracts up to `n` outputs from `exp`.
+          The `limit` function extracts up to `n` outputs from `expr`.
 
         examples:
-          - program: '[limit(3;.[])]'
+          - program: '[limit(3; .[])]'
             input: '[0,1,2,3,4,5,6,7,8,9]'
             output: ['[0,1,2]']
 
+      - title: "`skip(n; expr)`"
+        body: |
+
+          The `skip` function skips the first `n` outputs from `expr`.
+
+        examples:
+          - program: '[skip(3; .[])]'
+            input: '[0,1,2,3,4,5,6,7,8,9]'
+            output: ['[3,4,5,6,7,8,9]']
+
       - title: "`first(expr)`, `last(expr)`, `nth(n; expr)`"
         body: |
 

jq.1.prebuilt

@@ -1,5 +1,5 @@
 .
-.TH "JQ" "1" "August 2024" "" ""
+.TH "JQ" "1" "September 2024" "" ""
 .
 .SH "NAME"
 \fBjq\fR \- Command\-line JSON processor
@@ -3435,14 +3435,14 @@ jq \'isempty(\.[])\'
 .
 .IP "" 0
 .
-.SS "limit(n; exp)"
-The \fBlimit\fR function extracts up to \fBn\fR outputs from \fBexp\fR\.
+.SS "limit(n; expr)"
+The \fBlimit\fR function extracts up to \fBn\fR outputs from \fBexpr\fR\.
 .
 .IP "" 4
 .
 .nf
 
-jq \'[limit(3;\.[])]\'
+jq \'[limit(3; \.[])]\'
    [0,1,2,3,4,5,6,7,8,9]
 => [0,1,2]
 .
@@ -3450,6 +3450,21 @@ jq \'[limit(3;\.[])]\'
 .
 .IP "" 0
 .
+.SS "skip(n; expr)"
+The \fBskip\fR function skips the first \fBn\fR outputs from \fBexpr\fR\.
+.
+.IP "" 4
+.
+.nf
+
+jq \'[skip(3; \.[])]\'
+   [0,1,2,3,4,5,6,7,8,9]
+=> [3,4,5,6,7,8,9]
+.
+.fi
+.
+.IP "" 0
+.
 .SS "first(expr), last(expr), nth(n; expr)"
 The \fBfirst(expr)\fR and \fBlast(expr)\fR functions extract the first and last values from \fBexpr\fR, respectively\.
 .

src/builtin.jq

@@ -149,10 +149,14 @@ def until(cond; next):
      def _until:
          if cond then . else (next|_until) end;
      _until;
-def limit($n; exp):
-    if $n > 0 then label $out | foreach exp as $item ($n; .-1; $item, if . <= 0 then break $out else empty end)
-    elif $n == 0 then empty
-    else exp end;
+def limit($n; expr):
+  if $n > 0 then label $out | foreach expr as $item ($n; . - 1; $item, if . <= 0 then break $out else empty end)
+  elif $n == 0 then empty
+  else error("limit doesn't support negative count") end;
+def skip($n; expr):
+  if $n > 0 then foreach expr as $item ($n; . - 1; if . < 0 then $item else empty end)
+  elif $n == 0 then expr
+  else error("skip doesn't support negative count") end;
 # range/3, with a `by` expression argument
 def range($init; $upto; $by):
     if $by > 0 then $init|while(. < $upto; . + $by)
@@ -169,7 +173,7 @@ 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 $out else empty end) end;
+  else first(skip($n; g)) end;
 def first: .[0];
 def last: .[-1];
 def nth($n): .[$n];

tests/jq.test

@@ -334,6 +334,30 @@ null
 "badness"
 [1]
 
+try limit(-1; error) catch .
+null
+"limit doesn't support negative count"
+
+[skip(3; .[])]
+[1,2,3,4,5,6,7,8,9]
+[4,5,6,7,8,9]
+
+[skip(0,2,3,4; .[])]
+[1,2,3]
+[1,2,3,3]
+
+[skip(3; .[])]
+[]
+[]
+
+try skip(-1; error) catch .
+null
+"skip doesn't support negative count"
+
+nth(1; 0,1,error("foo"))
+null
+1
+
 [first(range(.)), last(range(.))]
 10
 [0,9]

tests/man.test

@@ -873,10 +873,14 @@ isempty(.[])
 [1,2,3]
 false
 
-[limit(3;.[])]
+[limit(3; .[])]
 [0,1,2,3,4,5,6,7,8,9]
 [0,1,2]
 
+[skip(3; .[])]
+[0,1,2,3,4,5,6,7,8,9]
+[3,4,5,6,7,8,9]
+
 [first(range(.)), last(range(.)), nth(./2; range(.))]
 10
 [0,9,5]