README.md

@@ -180,9 +180,9 @@ many times — each an independent draw — joined by `separator` (default `""`)
 { "format": "{word}", "repeat": 3, "separator": " ", "word": ["foo", "bar", "baz"] }
 ```
 
-Renders e.g. `bar foo baz`. A `separator` without a `repeat` is rejected, and so
+Renders e.g. `bar foo baz`. Rejected at load: a `separator` without a `repeat`,
-is a `repeat` that multiplies to more than 1 048 576 renders along any path of
+a `separator` of `""` (the default), and a `repeat` that multiplies to more than
-nested repeats.
+1 048 576 renders along any path of nested repeats.
 
 ### Options and fields
 
@@ -194,9 +194,11 @@ choice naming its item.
 ### Functions
 
 A `{name(args)}` token calls a builtin. Arguments are checked at `New`: a bad
-count, range, country or expression fails fast, and a length, count or decimal
+count, range, country or expression fails fast; an integer is written plain
-place beyond a sane maximum is rejected, so a fat-fingered `hex(2000000000)`
+(`5`, not `+5` or `05`); bounds are finite; a sample that could only ever emit one
-never tries to allocate gigabytes. Every builtin draws only from the seed — a
+value (`int(5,5)`, `float(1,1,2)`) is rejected naming the text to write instead;
+and a length, count or decimal place beyond a sane maximum is rejected, so a
+fat-fingered `hex(2000000000)` never tries to allocate gigabytes. Every builtin draws only from the seed — a
 time-based id takes its timestamp from the rng, not the clock — so seeded output
 stays reproducible.
 
@@ -243,8 +245,9 @@ hyphenated field can't be an operand.
 ```
 
 Renders e.g. `19.99 x 3 = 59.97`. An operand that can never be a number (`"abc"`,
-or a choice of such) is rejected at load; one that sometimes is not yields `NaN`,
+or a choice of such) is rejected at load, as is a division by a constant zero
-and a division by zero `Inf` — both print rather than fail.
+(`1/0`, or a fixed `"0"` field); an operand that sometimes is not a number yields
+`NaN`, and a division by a sometimes-zero one `Inf` — both print rather than fail.
 
 ### Transforms
 

builtins.go

@@ -176,7 +176,7 @@ var asciiFolds = map[rune]string{
 func asciiFold(s string) string {
 	var b strings.Builder
 	for _, r := range s {
-		if r < unicode.MaxASCII {
+		if r <= unicode.MaxASCII {
 			b.WriteRune(r)
 		} else {
 			b.WriteString(asciiFolds[r])
@@ -221,10 +221,25 @@ func randChars(r rng, n int, alphabet string) string {
 	return string(b)
 }
 
+// plainInt parses an integer arg written the one way: no sign, no leading zero.
+func plainInt(s string) (int, error) {
+	n, err := strconv.Atoi(s)
+	if err != nil {
+		return 0, fmt.Errorf("%q is not an integer", s)
+	}
+	if strconv.Itoa(n) != s {
+		return 0, fmt.Errorf("%q is not a plain integer; write %d", s, n)
+	}
+	return n, nil
+}
+
 func posIntArg(_ map[string]node, a []string) error {
-	n, err := strconv.Atoi(a[0])
+	n, err := plainInt(a[0])
-	if err != nil || n < 1 {
+	if err != nil {
-		return fmt.Errorf("count %q must be a positive integer", a[0])
+		return fmt.Errorf("count %w", err)
+	}
+	if n < 1 {
+		return fmt.Errorf("count %q must be positive", a[0])
 	}
 	if n > maxLen {
 		return fmt.Errorf("count %d exceeds the maximum %d", n, maxLen)
@@ -233,14 +248,17 @@ func posIntArg(_ map[string]node, a []string) error {
 }
 
 func intRangeArgs(_ map[string]node, a []string) error {
-	lo, e1 := strconv.Atoi(a[0])
+	lo, e1 := plainInt(a[0])
-	hi, e2 := strconv.Atoi(a[1])
+	hi, e2 := plainInt(a[1])
 	if e1 != nil || e2 != nil {
-		return fmt.Errorf("int(min,max) needs integer args, got %q,%q", a[0], a[1])
+		return fmt.Errorf("int(min,max) needs plain integers, got %q,%q", a[0], a[1])
 	}
 	if lo > hi {
 		return fmt.Errorf("int(min,max): min %d > max %d", lo, hi)
 	}
+	if lo == hi {
+		return fmt.Errorf("int(%d,%d) is the constant %d; write it as text", lo, hi, lo)
+	}
 	if uint64(hi)-uint64(lo) >= uint64(math.MaxInt64) { // span hi-lo+1 would overflow int -> IntN panic
 		return fmt.Errorf("int(min,max): range %d..%d is too wide", lo, hi)
 	}
@@ -250,19 +268,25 @@ func intRangeArgs(_ map[string]node, a []string) error {
 func floatArgs(_ map[string]node, a []string) error {
 	lo, e1 := strconv.ParseFloat(a[0], 64)
 	hi, e2 := strconv.ParseFloat(a[1], 64)
-	dp, e3 := strconv.Atoi(a[2])
+	dp, e3 := plainInt(a[2])
 	if e1 != nil || e2 != nil || e3 != nil {
-		return fmt.Errorf("float(min,max,dp) needs numeric args, got %q,%q,%q", a[0], a[1], a[2])
+		return fmt.Errorf("float(min,max,dp) needs numeric bounds and a plain decimals count, got %q,%q,%q", a[0], a[1], a[2])
+	}
+	if math.IsNaN(lo) || math.IsNaN(hi) || math.IsInf(lo, 0) || math.IsInf(hi, 0) {
+		return fmt.Errorf("float(min,max,dp) needs finite bounds, got %q,%q", a[0], a[1])
 	}
 	if lo > hi {
 		return fmt.Errorf("float(min,max,dp): min %v > max %v", lo, hi)
 	}
-	if math.IsInf(hi-lo, 0) { // an overflowing span would render as "+Inf"
-		return fmt.Errorf("float(min,max,dp): range %v..%v is too wide", lo, hi)
-	}
 	if dp < 0 || dp > maxDecimals {
 		return fmt.Errorf("float(min,max,dp): decimals %d out of range 0..%d", dp, maxDecimals)
 	}
+	if lo == hi {
+		return fmt.Errorf("float(%s,%s,%d) is the constant %q; write it as text", a[0], a[1], dp, strconv.FormatFloat(lo, 'f', dp, 64))
+	}
+	if math.IsInf(hi-lo, 0) { // an overflowing span would render as "+Inf"
+		return fmt.Errorf("float(min,max,dp): range %v..%v is too wide", lo, hi)
+	}
 	return nil
 }
 

calc.go

@@ -78,14 +78,78 @@ func checkCalc(fields map[string]node, args []string) error {
 			return fmt.Errorf("calc(%q): operand %q is never a number: it renders %q", args[0], name, text)
 		}
 	}
+	if divisor, zero := constantZeroDivisor(expr, fields); zero {
+		return fmt.Errorf("calc(%q) divides by %s, which is always zero", args[0], divisor)
+	}
 	if len(args) == 2 {
-		if dp, err := strconv.Atoi(args[1]); err != nil || dp < 0 || dp > maxDecimals {
+		if dp, err := plainInt(args[1]); err != nil || dp < 0 || dp > maxDecimals {
-			return fmt.Errorf("calc decimals %q must be an integer in 0..%d", args[1], maxDecimals)
+			return fmt.Errorf("calc decimals %q must be a plain integer in 0..%d", args[1], maxDecimals)
 		}
 	}
 	return nil
 }
 
+// constantZeroDivisor finds a division whose right side is a constant zero: number
+// literals and fixed operands folded, anything that varies left unknown.
+func constantZeroDivisor(n calcNode, fields map[string]node) (string, bool) {
+	switch n := n.(type) {
+	case calcNeg:
+		return constantZeroDivisor(n.x, fields)
+	case calcBin:
+		if n.op == '/' {
+			if v, known := constantValue(n.r, fields); known && v == 0 {
+				return calcText(n.r), true
+			}
+		}
+		if d, zero := constantZeroDivisor(n.l, fields); zero {
+			return d, true
+		}
+		return constantZeroDivisor(n.r, fields)
+	}
+	return "", false
+}
+
+// constantValue evaluates an expression whose every operand is fixed.
+func constantValue(n calcNode, fields map[string]node) (float64, bool) {
+	switch n := n.(type) {
+	case calcNum:
+		return float64(n), true
+	case calcVar:
+		t, ok := fields[string(n)].(*template)
+		if !ok || !t.fixed || t.repeat > 1 {
+			return 0, false
+		}
+		v, err := strconv.ParseFloat(strings.TrimSpace(t.lit), 64)
+		return v, err == nil
+	case calcNeg:
+		v, ok := constantValue(n.x, fields)
+		return -v, ok
+	case calcBin:
+		l, lok := constantValue(n.l, fields)
+		r, rok := constantValue(n.r, fields)
+		if !lok || !rok {
+			return 0, false
+		}
+		return calcBin{n.op, calcNum(l), calcNum(r)}.eval(nil), true
+	}
+	return 0, false
+}
+
+// calcText spells an expression node the way an author would read it.
+func calcText(n calcNode) string {
+	switch n := n.(type) {
+	case calcNum:
+		return strconv.FormatFloat(float64(n), 'f', -1, 64)
+	case calcVar:
+		return string(n)
+	case calcNeg:
+		return "-" + calcText(n.x)
+	case calcBin:
+		return "(" + calcText(n.l) + " " + string(n.op) + " " + calcText(n.r) + ")"
+	}
+	return "?"
+}
+
 // neverNumeric reports a node no render of which is a number: fixed text that does
 // not parse, or a choice of only such items. text is one such render.
 func neverNumeric(n node) (text string, never bool) {

hold.go

@@ -273,6 +273,9 @@ type draws struct {
 // linkRefs prove every step, so the walk cannot fail.
 func readField(s *session, t *template, held *draws, a arm) string {
 	if !t.held[a.key] {
+		if len(a.tail) > 0 {
+			panic(fmt.Sprintf("fejkdata: %q reads a path into %q, which the expansion does not hold", a.name, a.key))
+		}
 		return render(s, t.fields[a.key])
 	}
 	if v, read := held.value[a.path]; read {

node.go

@@ -57,6 +57,15 @@ type template struct {
 
 func (*template) isNode() {}
 
+// field is the node a path segment names; a binding is a render edge, not a field.
+func (t *template) field(seg string) (node, bool) {
+	if isRef(seg) {
+		return nil, false
+	}
+	n, ok := t.fields[seg]
+	return n, ok
+}
+
 // compile converts parsed JSON into a node tree, validating structure up front.
 // Only a choice's items carry a weight, so one here would be inert whatever its type.
 func compile(v any) (node, error) {
@@ -229,6 +238,9 @@ func readOptions(m map[string]any) (templateOptions, error) {
 		if repeat == 1 {
 			return o, fmt.Errorf("separator joins repeated renders, so it has no effect without a repeat above 1")
 		}
+		if o.separator == "" {
+			return o, fmt.Errorf("separator \"\" is the default, so it has no effect; drop it")
+		}
 	}
 	_, o.weighted = m["weight"]
 	return o, nil

path.go

@@ -40,7 +40,7 @@ func walkPath(n node, tail []string, w pathWalk) error {
 				return err
 			}
 		}
-		child, ok := n.fields[tail[0]]
+		child, ok := n.field(tail[0])
 		if !ok {
 			return fmt.Errorf("no field %q", tail[0])
 		}