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Use a general Eval function for expressions in templates. (#23927)

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One of the proposals in #23328

This PR introduces a simple expression calculator
(templates/eval/eval.go), it can do basic expression calculations.

Many untested template helper functions like `Mul` `Add` can be replaced
by this new approach.

Then these `Add` / `Mul` / `percentage` / `Subtract` / `DiffStatsWidth`
could all use this `Eval`.

And it provides enhancements for Golang templates, and improves
readability.

Some examples:

----

* Before: `{{Add (Mul $glyph.Row 12) 12}}`
* After: `{{Eval $glyph.Row "*" 12 "+" 12}}`

----

* Before: `{{if lt (Add $i 1) (len $.Topics)}}`
* After: `{{if Eval $i "+" 1 "<" (len $.Topics)}}`

## FAQ

### Why not use an existing expression package?

We need a highly customized expression engine:

* do the calculation on the fly, without pre-compiling
* deal with int/int64/float64 types, to make the result could be used in
Golang template.
* make the syntax could be used in the Golang template directly
* do not introduce too much complex or strange syntax, we just need a
simple calculator.
* it needs to strictly follow Golang template's behavior, for example,
Golang template treats all non-zero values as truth, but many 3rd
packages don't do so.

### What's the benefit?

* Developers don't need to add more `Add`/`Mul`/`Sub`-like functions,
they were getting more and more.
Now, only one `Eval` is enough for all cases.
* The new code reads better than old `{{Add (Mul $glyph.Row 12) 12}}`,
the old one isn't familiar to most procedural programming developers
(eg, the Golang expression syntax).
* The `Eval` is fully covered by tests, many old `Add`/`Mul`-like
functions were never tested.

### The performance?

It doesn't use `reflect`, it doesn't need to parse or compile when used
in Golang template, the performance is as fast as native Go template.

### Is it too complex? Could it be unstable?

The expression calculator program is a common homework for computer
science students, and it's widely used as a teaching and practicing
purpose for developers. The algorithm is pretty well-known.

The behavior can be clearly defined, it is stable.
This commit is contained in:
wxiaoguang
2023-04-07 21:25:49 +08:00
committed by GitHub
parent ecf34fcd89
commit 5b89670a31
15 changed files with 529 additions and 157 deletions
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39
modules/base/tool_test.go
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@ -114,45 +114,6 @@ func TestFileSize(t *testing.T) {
assert.Equal(t, "2.0 EiB", FileSize(size))
}
func TestSubtract(t *testing.T) {
toFloat64 := func(n interface{}) float64 {
switch v := n.(type) {
case int:
return float64(v)
case int8:
return float64(v)
case int16:
return float64(v)
case int32:
return float64(v)
case int64:
return float64(v)
case float32:
return float64(v)
case float64:
return v
default:
return 0.0
}
}
values := []interface{}{
int(-3),
int8(14),
int16(81),
int32(-156),
int64(1528),
float32(3.5),
float64(-15.348),
}
for _, left := range values {
for _, right := range values {
expected := toFloat64(left) - toFloat64(right)
sub := Subtract(left, right)
assert.InDelta(t, expected, sub, 1e-3)
}
}
}
func TestEllipsisString(t *testing.T) {
assert.Equal(t, "...", EllipsisString("foobar", 0))
assert.Equal(t, "...", EllipsisString("foobar", 1))