fix: preserve numerical precision on unary negation

docs/content/manual/dev/manual.yml

@@ -373,10 +373,7 @@ sections:
           such `1E1234567890`, precision will be lost if the exponent
           is too large.
 
-          (3) In jq programs, a leading minus sign will trigger the
-          conversion of the number to an IEEE754 representation.
-
-          (4) Comparisons are carried out using the untruncated
+          (3) Comparisons are carried out using the untruncated
           big decimal representation of numbers if available, as
           illustrated in one of the following examples.
 
@@ -395,15 +392,19 @@ sections:
             input: '0.12345678901234567890123456789'
             output: ['0.12345678901234567890123456789']
 
-          - program: '[., tojson] | . == if have_decnum then [12345678909876543212345,"12345678909876543212345"] else [12345678909876543000000,"12345678909876543000000"] end'
+          - program: '[., tojson] == if have_decnum then [12345678909876543212345,"12345678909876543212345"] else [12345678909876543000000,"12345678909876543000000"] end'
             input: '12345678909876543212345'
             output: ['true']
 
+          - program: '[1234567890987654321,-1234567890987654321 | tojson] == if have_decnum then ["1234567890987654321","-1234567890987654321"] else ["1234567890987654400","-1234567890987654400"] end'
+            input: 'null'
+            output: ['true']
+
           - program: '. < 0.12345678901234567890123456788'
             input: '0.12345678901234567890123456789'
             output: ['false']
 
-          - program: 'map([., . == 1]) | tojson | . == if have_decnum then "[[1,true],[1.000,true],[1.0,true],[1.00,true]]" else "[[1,true],[1,true],[1,true],[1,true]]" end'
+          - program: 'map([., . == 1]) | tojson == if have_decnum then "[[1,true],[1.000,true],[1.0,true],[1.00,true]]" else "[[1,true],[1,true],[1,true],[1,true]]" end'
             input: '[1, 1.000, 1.0, 100e-2]'
             output: ['true']
 

src/builtin.c

@@ -256,7 +256,7 @@ static jv f_negate(jq_state *jq, jv input) {
   if (jv_get_kind(input) != JV_KIND_NUMBER) {
     return type_error(input, "cannot be negated");
   }
-  jv ret = jv_number(-jv_number_value(input));
+  jv ret = jv_number_negate(input);
   jv_free(input);
   return ret;
 }

src/jv.c

@@ -562,7 +562,6 @@ static decNumber* jvp_dec_number_ptr(jv j) {
 }
 
 static jvp_literal_number* jvp_literal_number_alloc(unsigned literal_length) {
-
   /* The number of units needed is ceil(DECNUMDIGITS/DECDPUN)         */
   int units = ((literal_length+DECDPUN-1)/DECDPUN);
 
@@ -571,19 +570,18 @@ static jvp_literal_number* jvp_literal_number_alloc(unsigned literal_length) {
     + sizeof(decNumberUnit) * units
   );
 
+  n->refcnt = JV_REFCNT_INIT;
+  n->num_double = NAN;
+  n->literal_data = NULL;
   return n;
 }
 
 static jv jvp_literal_number_new(const char * literal) {
+  jvp_literal_number* n = jvp_literal_number_alloc(strlen(literal));
 
-  jvp_literal_number * n = jvp_literal_number_alloc(strlen(literal));
-
-  n->refcnt = JV_REFCNT_INIT;
-  n->literal_data = NULL;
   decContext *ctx = DEC_CONTEXT();
   decContextClearStatus(ctx, DEC_Conversion_syntax);
   decNumberFromString(&n->num_decimal, literal, ctx);
-  n->num_double = NAN;
 
   if (ctx->status & DEC_Conversion_syntax) {
     jv_mem_free(n);
@@ -734,6 +732,21 @@ int jvp_number_is_nan(jv n) {
   return n.u.number != n.u.number;
 }
 
+jv jv_number_negate(jv n) {
+  assert(JVP_HAS_KIND(n, JV_KIND_NUMBER));
+
+#ifdef USE_DECNUM
+  if (JVP_HAS_FLAGS(n, JVP_FLAGS_NUMBER_LITERAL)) {
+    jvp_literal_number* m = jvp_literal_number_alloc(jvp_dec_number_ptr(n)->digits);
+
+    decNumberMinus(&m->num_decimal, jvp_dec_number_ptr(n), DEC_CONTEXT());
+    jv r = {JVP_FLAGS_NUMBER_LITERAL, 0, 0, 0, {&m->refcnt}};
+    return r;
+  }
+#endif
+  return jv_number(-jv_number_value(n));
+}
+
 int jvp_number_cmp(jv a, jv b) {
   assert(JVP_HAS_KIND(a, JV_KIND_NUMBER));
   assert(JVP_HAS_KIND(b, JV_KIND_NUMBER));

src/jv.h

@@ -74,8 +74,9 @@ jv jv_number(double);
 jv jv_number_with_literal(const char*);
 double jv_number_value(jv);
 int jv_is_integer(jv);
+jv jv_number_negate(jv);
 
-int jv_number_has_literal(jv n);
+int jv_number_has_literal(jv);
 const char* jv_number_get_literal(jv);
 
 jv jv_array(void);

tests/jq.test

@@ -1985,6 +1985,19 @@ true
 {"x":13911860366432393}
 13911860366432382
 
+# Unary negation preserves numerical precision
+-. | tojson == if have_decnum then "-13911860366432393" else "-13911860366432392" end
+13911860366432393
+true
+
+-. | tojson == if have_decnum then "0.12345678901234567890123456789" else "0.12345678901234568" end
+-0.12345678901234567890123456789
+true
+
+[1E+1000,-1E+1000 | tojson] == if have_decnum then ["1E+1000","-1E+1000"] else ["1.7976931348623157e+308","-1.7976931348623157e+308"] end
+null
+true
+
 . |= try . catch .
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