fix(json): preserve floats with trailing zero when encoding YAML to JSON (#2701)

YAML scalars tagged `!!float` were round-tripped through `float64` and
re-serialized by Go's JSON encoder, which strips the decimal part of
whole-number floats. As a result, `50.0` came out as `50` and a
sequence like `[50.0, 95.0, 99.0, 99.9]` became `[50,95,99,99.9]`,
turning a uniform array of floats into a mixed int/float array that
downstream consumers (Horreum, JSON Schema validators, jq, etc.)
reject.

The JSON spec does not distinguish ints from floats, but every common
JSON library (Go's `encoding/json`, Python's `json`, jq) preserves the
fractional form of values that came in as floats. yq's YAML decoder
already parses these as `!!float` with the original text intact, so we
can emit them verbatim instead of round-tripping.

`MarshalJSON` for `ScalarNode` now special-cases `!!float`:
- if `Value` is already a JSON-shaped number literal containing a `.`
  or exponent, emit it verbatim (e.g. `50.0`, `99.9`, `1.5e-3`, `-7.0`);
- if `Value` is an integer-shaped string tagged `!!float` (e.g.
  `!!float 5`), format the parsed float and append `.0` so it stays a
  JSON number with a fractional part;
- otherwise (empty value, parse error, or non-finite result), fall back
  to the existing encoding path so behaviour for `.inf` / `.nan` and
  anything unusual is unchanged.

`!!int` nodes still encode as JSON integers.

Closes #2683

Signed-off-by: ChrisJr404 <chris@hacknow.com>

pkg/yqlib/candidate_node_json.go

@@ -7,6 +7,9 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
+	"strconv"
+	"strings"
 
 	"github.com/goccy/go-json"
 )
@@ -140,6 +143,12 @@ func (o *CandidateNode) MarshalJSON() ([]byte, error) {
 		return buf.Bytes(), err
 	case ScalarNode:
 		log.Debugf("MarshalJSON ScalarNode")
+		if o.guessTagFromCustomType() == "!!float" {
+			if raw, ok := jsonFloatLiteral(o.Value); ok {
+				buf.WriteString(raw)
+				return buf.Bytes(), nil
+			}
+		}
 		value, err := o.GetValueRep()
 		if err != nil {
 			return buf.Bytes(), err
@@ -177,3 +186,85 @@ func (o *CandidateNode) MarshalJSON() ([]byte, error) {
 		return buf.Bytes(), err
 	}
 }
+
+// jsonFloatLiteral returns a JSON-shaped representation of a YAML !!float scalar
+// value, preserving the original textual form (e.g. "50.0" stays "50.0") whenever
+// possible. The second return value is false when the value cannot be safely
+// rendered as a JSON number (e.g. ".inf", ".nan", or anything that parses to a
+// non-finite float); callers should fall back to the normal encoding path in
+// that case, which preserves the existing behaviour for those inputs.
+func jsonFloatLiteral(raw string) (string, bool) {
+	if raw == "" {
+		return "", false
+	}
+	f, err := strconv.ParseFloat(raw, 64)
+	if err != nil {
+		return "", false
+	}
+	if math.IsInf(f, 0) || math.IsNaN(f) {
+		return "", false
+	}
+	if isJSONNumberLiteral(raw) {
+		return raw, true
+	}
+	formatted := strconv.FormatFloat(f, 'f', -1, 64)
+	if !strings.ContainsAny(formatted, ".eE") {
+		formatted += ".0"
+	}
+	return formatted, true
+}
+
+// isJSONNumberLiteral reports whether s is already a valid JSON number literal
+// representing a fractional value (i.e. contains a "." or an exponent), so it
+// can be emitted verbatim without round-tripping through a float64.
+func isJSONNumberLiteral(s string) bool {
+	if s == "" {
+		return false
+	}
+	i := 0
+	if s[i] == '-' {
+		i++
+		if i == len(s) {
+			return false
+		}
+	}
+	// integer part: 0 or [1-9][0-9]*
+	if s[i] == '0' {
+		i++
+	} else if s[i] >= '1' && s[i] <= '9' {
+		for i < len(s) && s[i] >= '0' && s[i] <= '9' {
+			i++
+		}
+	} else {
+		return false
+	}
+	hasFraction := false
+	if i < len(s) && s[i] == '.' {
+		hasFraction = true
+		i++
+		if i == len(s) || s[i] < '0' || s[i] > '9' {
+			return false
+		}
+		for i < len(s) && s[i] >= '0' && s[i] <= '9' {
+			i++
+		}
+	}
+	hasExponent := false
+	if i < len(s) && (s[i] == 'e' || s[i] == 'E') {
+		hasExponent = true
+		i++
+		if i < len(s) && (s[i] == '+' || s[i] == '-') {
+			i++
+		}
+		if i == len(s) || s[i] < '0' || s[i] > '9' {
+			return false
+		}
+		for i < len(s) && s[i] >= '0' && s[i] <= '9' {
+			i++
+		}
+	}
+	if i != len(s) {
+		return false
+	}
+	return hasFraction || hasExponent
+}