README.md

@@ -335,16 +335,20 @@ container.
   refused, exact powers are capped, and every number is held as a
   fraction, whole numbers too, under the 4096-bit limit below, so a
   message cannot make the bot do unbounded work. Whole numbers below
-  10<sup>21</sup> are written exactly; other results in the shortest
+  10<sup>21</sup> are written exactly. Other results inside the normal
-  form that reads back as the same double, in exponent notation from
+  range of a double, about 2.2e-308 to 1.8e308 in magnitude, where a
-  10<sup>21</sup> up and below 10<sup>-6</sup>. Refused as too large or
+  double keeps all its digits, are written in the shortest form that
-  too small: any number whose numerator or denominator reaches 4096
+  reads back as the same double, in exponent notation from
-  bits, wherever it appears, as `go/constant` rounds a fraction that
+  10<sup>21</sup> up and below 10<sup>-6</sup>. Results outside that
-  grows that large (`1e-1300 + 1`); a power computed as a double whose
+  range are written from their exact value in exponent notation, rounded
-  base or result is outside the normal range of a double, about 2.2e-308
+  to 17 significant digits, the most the shortest form of a double
-  to 1.8e308 in magnitude, where a double keeps all its digits
+  takes, with trailing zeros dropped: `2^1200` is
-  (`1e-400^0.5`); and a result other than zero outside that range, as it
+  `1.7218479456385751e+361`, `10^400` is `1e+400` and `2^-1074` is
-  is written through a double (`1e400`, `2^-1400`).
+  `4.9406564584124654e-324`. Refused as too large or too small: any
+  number whose numerator or denominator reaches 4096 bits, wherever it
+  appears, as `go/constant` rounds a fraction that grows that large
+  (`2^4095`, `1e-1300 + 1`); and a power computed as a double whose base
+  or result is outside the normal range of a double (`1e-400^0.5`).
 - **Failure is an exit.** If the chat client exits or the connection to
   it drops, the bot exits with an error and the container's restart
   policy starts both again. `SIGTERM` stops the bot, which stops the

docs/TODO.md

@@ -27,6 +27,8 @@ with no deprecation warning.
 
 # Completed Steps
 
+- 2026-09-29 Exact results past the range of a double, such as `2^1200`,
+  are written to 17 significant digits instead of being refused
 - 2026-09-29 `GET` and `POST /api/v1/chats/{id}/messages`: a chat's
   latest messages, and sending one
 - 2026-09-29 Added the HTTP API in `internal/api`: the server on `PORT`,

internal/bot/bot_test.go

@@ -18,6 +18,7 @@ func TestReply(t *testing.T) {
 		"5 * 5/2": "12.5",
 		"2^10":    "1024",
 		"7 % 3":   "1",
+		"2^1200":  "1.7218479456385751e+361",
 	} {
 		if got := bot.Reply(in); got != want {
 			t.Errorf("Reply(%q) = %q, want %q", in, got, want)
@@ -27,7 +28,7 @@ func TestReply(t *testing.T) {
 	for in, want := range map[string]string{
 		"hello":    "I only understand arithmetic",
 		"1 / 0":    "I cannot divide by zero.",
-		"1e400":    "That needs a number too large or too small for me.",
+		"1e1300":   "That needs a number too large or too small for me.",
 		"1e-1300":  "That needs a number too large or too small for me.",
 		"(-8)^0.5": "A negative number to a fractional power has no real",
 		strings.Repeat("1+", calc.MaxInputLength) + "1": "That is too long for me",

internal/calc/calc.go

@@ -48,6 +48,16 @@ const (
 	plainLower = 1e-6
 )
 
+// A result past the normal range of a double is written to
+// significantDigits significant digits, the most the shortest form of a
+// double takes. It is rounded to them from a float of floatPrecision
+// bits, the bits a numerator or denominator can hold and 64 more for the
+// digits, so that the float rounds as the exact result would.
+const (
+	significantDigits = 17
+	floatPrecision    = bitLimit + 64
+)
+
 // The precedence of the binary operators: the higher, the tighter the
 // operator binds.
 const (
@@ -82,8 +92,7 @@ var (
 )
 
 // Evaluate computes an arithmetic expression and returns its result as
-// text: whole numbers without a decimal point, fractions in the
+// text, written as format describes.
-// shortest form that reads back as the same float64.
 func Evaluate(input string) (string, error) {
 	s := strings.TrimSpace(input)
 	if len(s) > MaxInputLength {
@@ -106,7 +115,7 @@ func Evaluate(input string) (string, error) {
 		return "", ErrNotArithmetic
 	}
 
-	return format(v)
+	return format(v), nil
 }
 
 // tokenize splits an expression into operators, parentheses and
@@ -459,26 +468,31 @@ func normal(f float64) bool {
 }
 
 // format writes a result for a person to read. A whole number of
-// ordinary size is written exactly, digit for digit; anything else goes
+// ordinary size is written exactly, digit for digit. Any other result in
-// through float64, whose shortest round-trip form is free of the noise
+// the normal range of a double goes through float64, whose shortest
-// (0.30000000000000004) that printing a binary fraction to a fixed
+// round-trip form is free of the noise (0.30000000000000004) that
-// precision produces. A result that is not zero must therefore be a
+// printing a binary fraction to a fixed precision produces. Past that
-// normal double: 2^-1074 would be written 5e-324.
+// range a double keeps fewer digits, or none (2^-1074 would be written
-func format(v constant.Value) (string, error) {
+// 5e-324, and 2^1024 is infinite), so such a result is written from its
+// exact value, to significantDigits.
+func format(v constant.Value) string {
 	f, _ := constant.Float64Val(v)
 	if constant.Sign(v) != 0 && !normal(f) {
-		return "", ErrOutOfRange
+		// Every number here is exact: see exact.
+		r, _ := constant.Val(v).(*big.Rat)
+
+		return new(big.Float).SetPrec(floatPrecision).SetRat(r).Text('g', significantDigits)
 	}
 
 	abs := math.Abs(f)
 
 	if i := constant.ToInt(v); i.Kind() == constant.Int && abs < plainUpper {
-		return i.ExactString(), nil
+		return i.ExactString()
 	}
 
 	if abs >= plainUpper || abs < plainLower {
-		return strconv.FormatFloat(f, 'g', -1, 64), nil
+		return strconv.FormatFloat(f, 'g', -1, 64)
 	}
 
-	return strconv.FormatFloat(f, 'f', -1, 64), nil
+	return strconv.FormatFloat(f, 'f', -1, 64)
 }

internal/calc/calc_test.go

@@ -52,6 +52,13 @@ func TestEvaluate(t *testing.T) {
 		"1234567.5":               "1234567.5",
 		"-1 / 4":                  "-0.25",
 		"1e300 * 1e8":             "1e+308",
+		// Past the normal range of a double, written from the exact value
+		// to 17 significant digits, trailing zeros dropped.
+		"1e400":         "1e+400",
+		"1e300 * 1e300": "1e+600",
+		"1 / 2e-400":    "5e+399",
+		"-1e-310":       "-1e-310",
+		"1 / 3e400":     "3.3333333333333333e-401",
 	})
 }
 
@@ -82,6 +89,7 @@ func TestEvaluatePowers(t *testing.T) {
 		"0.1^2":             "0.01",
 		"2^100 - 2^100 + 1": "1",
 		"2^64":              "18446744073709551616",
+		"2^100":             "1.2676506002282294e+30",
 		"0^0":               "1",
 		"0^3":               "0",
 		"1.5^2":             "2.25",
@@ -99,6 +107,29 @@ func TestEvaluatePowers(t *testing.T) {
 		"2^-1400 * 2^1365 * 2^35": "1",
 		"0.3^900 * 10^470":        "0.25652473503365386",
 		"2^1500 / 2^1000":         "3.273390607896142e+150",
+		"(2^1200)/(2^1199)":       "2",
+		"2^1200 % 7":              "1",
+		// Results past the range of a double, written to 17 significant
+		// digits, up to the largest power of 2 under the 4096-bit limit.
+		"2^1200":          "1.7218479456385751e+361",
+		"2**1200":         "1.7218479456385751e+361",
+		"2^1024":          "1.7976931348623159e+308",
+		"10^400":          "1e+400",
+		"2^-1074":         "4.9406564584124654e-324",
+		"2^-1200":         "5.8077137562175032e-362",
+		"2^-1400":         "3.6141491434385841e-422",
+		"0.5^1100":        "7.3621518290228627e-332",
+		"1.5^2000":        "1.5223626185737825e+352",
+		"(1/3)^-2000":     "1.7478712517226516e+954",
+		"2^1200 - 2^1199": "8.6092397281928753e+360",
+		"-2^1201":         "-3.4436958912771501e+361",
+		"(-2)^1201":       "-3.4436958912771501e+361",
+		"2^2000 * 2^2000": "1.3182040934309431e+1204",
+		"10^1232":         "1e+1232",
+		"2^4094":          "2.6109722035328813e+1232",
+		// A power computed in float64 carries its rounding into the exact
+		// arithmetic after it, past the range of a double as within it.
+		"2^0.5 * 1e400": "1.4142135623730951e+400",
 	})
 }
 
@@ -205,25 +236,22 @@ func TestEvaluateRefuses(t *testing.T) {
 }
 
 // TestEvaluateOutOfRange: a number is held exactly, or computed in
-// float64 as a normal double, and a result is written as a normal
+// float64 as a normal double. Anything else is refused.
-// double. Anything else is refused.
 func TestEvaluateOutOfRange(t *testing.T) {
 	t.Parallel()
 
 	expectErrors(t, map[string]error{
-		// Results that are not normal doubles: 2^-1074 would be written
+		// Just past the 4096-bit limit, which 2^4094 and 10^1232 are
-		// 5e-324.
+		// under, and far past it.
-		"1e400":                     calc.ErrOutOfRange,
+		"2^4095":                    calc.ErrOutOfRange,
-		"1e300 * 1e300":             calc.ErrOutOfRange,
+		"-2^4095":                   calc.ErrOutOfRange,
+		"2^-4095":                   calc.ErrOutOfRange,
+		"10^1233":                   calc.ErrOutOfRange,
+		"2^4094 * 2":                calc.ErrOutOfRange,
 		"1e999999999 * 1e999999999": calc.ErrOutOfRange,
-		"1 / 1e-400":                calc.ErrOutOfRange,
-		"2^1024":                    calc.ErrOutOfRange,
 		"2^5000":                    calc.ErrOutOfRange,
 		"(-2)^5001":                 calc.ErrOutOfRange,
 		"0.5^-5000":                 calc.ErrOutOfRange,
-		"2^-1074":                   calc.ErrOutOfRange,
-		"2^-1400":                   calc.ErrOutOfRange,
-		"-1e-310":                   calc.ErrOutOfRange,
 		// Powers computed in float64 whose base or result is not a
 		// normal double, and so has lost digits, or all of them.
 		"2^-1073.5 * 2^1073":          calc.ErrOutOfRange,
@@ -232,6 +260,7 @@ func TestEvaluateOutOfRange(t *testing.T) {
 		"1e-310^0.5":                  calc.ErrOutOfRange,
 		"(0.5^1100)^4 / (0.5^1100)^4": calc.ErrOutOfRange,
 		"(1/3)^1e400":                 calc.ErrOutOfRange,
+		"(2^1200)^0.5":                calc.ErrOutOfRange,
 		// go/constant holds numbers of this size rounded. A sum of them
 		// can lose the answer (this one would be 0), and so can a
 		// remainder or the sign of -1 to such a power.
@@ -309,6 +338,11 @@ func TestEvaluateBoundsWork(t *testing.T) {
 		// the limit, and a literal whose exponent is too large to read.
 		{in: "(3^2583/5^1760)^4096", err: calc.ErrOutOfRange},
 		{in: "1e99999999999999999999", err: calc.ErrOutOfRange},
+		// Results just below the limit, written from their exact value.
+		{in: "2^4094", want: "2.6109722035328813e+1232"},
+		{in: "3^2583", want: "2.5363018640659988e+1232"},
+		{in: "2^-4094", want: "3.8299909843808741e-1233"},
+		{in: "-1/3^2583", want: "-3.9427483540814775e-1233"},
 	}
 
 	for _, c := range cases {