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

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.

1 files changed, 0 insertions, 49 deletions

diff --git a/modules/base/tool.go b/modules/base/tool.go
index bd3a8458ee..13b07c043e 100644
--- a/modules/base/tool.go
+++ b/modules/base/tool.go
@@ -141,55 +141,6 @@ func FileSize(s int64) string {
 	return humanize.IBytes(uint64(s))
 }
 
-// Subtract deals with subtraction of all types of number.
-func Subtract(left, right interface{}) interface{} {
-	var rleft, rright int64
-	var fleft, fright float64
-	isInt := true
-	switch v := left.(type) {
-	case int:
-		rleft = int64(v)
-	case int8:
-		rleft = int64(v)
-	case int16:
-		rleft = int64(v)
-	case int32:
-		rleft = int64(v)
-	case int64:
-		rleft = v
-	case float32:
-		fleft = float64(v)
-		isInt = false
-	case float64:
-		fleft = v
-		isInt = false
-	}
-
-	switch v := right.(type) {
-	case int:
-		rright = int64(v)
-	case int8:
-		rright = int64(v)
-	case int16:
-		rright = int64(v)
-	case int32:
-		rright = int64(v)
-	case int64:
-		rright = v
-	case float32:
-		fright = float64(v)
-		isInt = false
-	case float64:
-		fright = v
-		isInt = false
-	}
-
-	if isInt {
-		return rleft - rright
-	}
-	return fleft + float64(rleft) - (fright + float64(rright))
-}
-
 // EllipsisString returns a truncated short string,
 // it appends '...' in the end of the length of string is too large.
 func EllipsisString(str string, length int) string {