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Added alternate Int36 addition/subtraction overflow/underflow detection logic as a verification check

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This commit is contained in:
Jeff Parsons 2017-02-16 14:05:05 -08:00 • committed by Jeff Parsons
commit 4de50acdf1
2 changed files with 85 additions and 26 deletions
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23
modules/shared/bin/int36
View file

@ -139,18 +139,17 @@ test("set 4001");
test("div -5");
test("set 0o037777777777 0o777777777777");
// test("set 0o047777777777 0o777777777777");
// test("set 0o057777777777 0o777777777777");
// test("set 0o067777777777 0o777777777777");
// test("set 0o077777777777 0o777777777777");
// test("set 0o177777777777 0o777777777777");
// test("set 0o277777777777 0o777777777777");
// test("set 0o377777777777 0o777777777777");
// test("set 0o477777777777 0o777777777777");
// test("set 0o577777777777 0o777777777777");
// test("set 0o677777777777 0o777777777777");
// test("set 0o777777777777 0o777777777777");
test("set 0o047777777777 0o777777777777");
test("set 0o057777777777 0o777777777777");
test("set 0o067777777777 0o777777777777");
test("set 0o077777777777 0o777777777777");
test("set 0o177777777777 0o777777777777");
test("set 0o277777777777 0o777777777777");
test("set 0o377777777777 0o777777777777");
test("set 0o477777777777 0o777777777777");
test("set 0o577777777777 0o777777777777");
test("set 0o677777777777 0o777777777777");
test("set 0o777777777777 0o777777777777");
repl.start({
prompt: "int36> ",

88
modules/shared/lib/int36.js
View file

@ -78,11 +78,11 @@ class Int36 {
*
* -Math.pow(2, 53) <= i <= Math.pow(2, 53)
*
* it seems unwise to ever permit the internal value to creep outside the 36-bit range, because
* it seems unwise to ever permit our internal values to creep outside the 36-bit range, because
* floating-point operations will drop least-significant bits in favor of most-significant bits when a
* result becomes too large, which is the opposite of what integer operations traditionally do. There
* might be some optimization benefits to performing our internal 36-bit truncation "lazily", but at
* least initially, I prefer to truncate the results of all operations immediately.
* least initially, I prefer to truncate() the results of all 36-bit arithmetic operations immediately.
*
* Most of the Int36 operations come in two flavors: those that accept numbers, and those that accept
* another Int36. The latter are more efficient, because (as just explained) an Int36's internal value
@ -209,25 +209,53 @@ class Int36 {
}
/**
* truncate(result, original)
* truncate(result, operand, fSub)
*
* The range of valid results (0 - MAXVAL) is divided into two equal sub-ranges: 0 to MAXPOS,
* where the sign bit is zero (the bottom range), and MAXPOS+1 to MAXVAL (the top range), where
* the sign bit is one. During a single arithmetic operation, the result can "wrap around" from
* the bottom range to the top, or from the top range to the bottom, but it's an overflow/underflow
* condition ONLY if the result "wraps across" the midpoint between the two ranges, producing an
* error ONLY if the result "wraps across" the midpoint between the two ranges, producing an
* unnaturally small delta (<= MAXPOS).
*
* This can be confirmed independently by examining the sign bits (BIT35) of the original value
* (V), the operand (O), the result (R), as well as two intermediate calculations, VR = (V ^ R)
* and OR = (O ^ R), and a final calculation: E = (VR & OR). In the case of subtraction (fSub),
* OV replaces OR.
*
* V O R VR OR E
* - - - -- -- -
* 0 0 0 0 0 0
* 0 0 1 1 1 1 (adding positive to positive yielded negative: overflow)
* 0 1 0 0 1 0
* 0 1 1 1 0 0
* 1 0 0 1 0 0
* 1 0 1 0 1 0
* 1 1 0 1 1 1 (adding negative to negative yielded positive: underflow)
* 1 1 1 0 0 0
*
* V O R VR OV E
* - - - -- -- -
* 0 0 0 0 0 0
* 0 0 1 1 0 0
* 0 1 0 0 1 0
* 0 1 1 1 1 1 (subtracting negative from positive yielded negative: overflow)
* 1 0 0 1 1 1 (subtracting positive from negative yielded positive: underflow)
* 1 0 1 0 1 0
* 1 1 0 1 0 0
* 1 1 1 0 0 0
*
* NOTE: This function's job is to truncate the result of an operation to 36-bit accuracy,
* not to remove any fractional portion that might also exist. If an operation could have produced
* a non-integer result (eg, div()), it's the caller's responsibility to deal with that first.
*
* @this {Int36}
* @param {number} result
* @param {number} [original]
* @param {number} [operand]
* @param {boolean} [fSub] (true if operand was subtracted)
* @return {number}
*/
truncate(result, original)
truncate(result, operand, fSub)
{
if (DEBUG && result !== Math.trunc(result)) {
console.log("Int36.truncate(" + result + " is not an integer)");
@ -243,11 +271,33 @@ class Int36 {
result %= Int36.BIT36;
if (original !== undefined && (result > Int36.MAXPOS) != (original > Int36.MAXPOS)) {
var delta = result - original;
if (Math.abs(delta) <= Int36.MAXPOS) {
this.error |= (delta > 0? Int36.ERROR.OVERFLOW : Int36.ERROR.UNDERFLOW);
/*
* We don't actually need to know what the operand was to determine overflow or underflow
* for addition or subtraction, just the original value (this.value) the new value (result).
*
* We do, however, need to know the operand if we want to confirm our error calculation using
* the truth table above, which requires examining the sign bits of all the inputs and outputs.
*/
if (operand !== undefined) {
/*
* Calculating V, R, O, and E as described above is somewhat tedious, because bits
* above bit 31 cannot be accessed directly; we shift all the sign bits down to bit 0
* using division first. We don't need to truncate the results, because the subsequent
* bit-wise operations perform truncation automatically.
*/
var e = 0;
if (DEBUG) {
var v = this.value / Int36.BIT35, r = result / Int36.BIT35, o = operand / Int36.BIT35;
e = ((v ^ r) & (o ^ (fSub? v : r))) & 1;
}
if ((result > Int36.MAXPOS) != (this.value > Int36.MAXPOS)) {
var delta = result - this.value;
if (Math.abs(delta) <= Int36.MAXPOS) {
this.error |= (delta > 0 ? Int36.ERROR.OVERFLOW : Int36.ERROR.UNDERFLOW);
if (DEBUG && (delta > 0) != !(e & v)) e = 0;
}
}
if (DEBUG && (!this.error) != (!e)) console.log("overflow mismatch");
}
return result;
}
@ -260,7 +310,7 @@ class Int36 {
*/
add(i36)
{
this.value = this.truncate(this.value + i36.value, this.value);
this.value = this.truncate(this.value + i36.value, i36.value);
}
/**
@ -271,7 +321,8 @@ class Int36 {
*/
addNum(num)
{
this.value = this.truncate(this.value + Int36.validate(num), this.value);
num = Int36.validate(num);
this.value = this.truncate(this.value + num, num);
}
/**
@ -282,7 +333,7 @@ class Int36 {
*/
sub(i36)
{
this.value = this.truncate(this.value - i36.value, this.value);
this.value = this.truncate(this.value - i36.value, i36.value, true);
}
/**
@ -293,7 +344,8 @@ class Int36 {
*/
subNum(num)
{
this.value = this.truncate(this.value - Int36.validate(num), this.value);
num = Int36.validate(num);
this.value = this.truncate(this.value - num, num, true);
}
/**
@ -599,11 +651,18 @@ class Int36 {
/**
* validate(num)
*
* This ensures that any incoming (external) 36-bit values conform to our internal requirements.
*
* @param {number} num
* @return {number}
*/
static validate(num)
{
/*
* Although it's expected that most callers will supply unsigned 36-bit values, we're nice about
* converting any signed values to their unsigned (two's complement) counterpart, provided they are
* within the acceptable range. Any signed values outside that range will be dealt with afterward.
*/
if (num < 0 && num >= Int36.MINNEG) {
num += Int36.BIT36;
}
@ -623,6 +682,7 @@ Int36.ERROR = {
};
Int36.BIT18 = Math.pow(2, 18); // 262,144
Int36.BIT35 = Math.pow(2, 35); // 34,359,738,368 (aka the sign bit)
Int36.BIT36 = Math.pow(2, 36); // 68,719,476,736
Int36.MAXPOS = Math.pow(2, 35) - 1; // 34,359,738,367