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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
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15 changed files with 529 additions and 157 deletions
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4
templates/repo/home.tmpl
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@ -37,7 +37,7 @@
<div class="ui form gt-hidden gt-df gt-mt-4" id="topic_edit">
<div class="field gt-f1 gt-mr-3">
<div class="ui fluid multiple search selection dropdown" data-text-count-prompt="{{.locale.Tr "repo.topic.count_prompt"}}" data-text-format-prompt="{{.locale.Tr "repo.topic.format_prompt"}}">
<input type="hidden" name="topics" value="{{range $i, $v := .Topics}}{{.Name}}{{if lt (Add $i 1) (len $.Topics)}},{{end}}{{end}}">
<input type="hidden" name="topics" value="{{range $i, $v := .Topics}}{{.Name}}{{if Eval $i "+" 1 "<" (len $.Topics)}},{{end}}{{end}}">
{{range .Topics}}
{{/* keey the same layout as Fomantic UI generated labels */}}
<a class="ui label transition visible gt-cursor-default" data-value="{{.Name}}" style="display: inline-block !important;">{{.Name}}{{svg "octicon-x" 16 "delete icon"}}</a>
@ -61,7 +61,7 @@
<div class="gt-df gt-ac gt-fw gt-gap-y-3">
{{template "repo/branch_dropdown" dict "root" .}}
{{$n := len .TreeNames}}
{{$l := Subtract $n 1}}
{{$l := Eval $n "-" 1}}
<!-- If home page, show new pr. If not, show breadcrumb -->
{{if eq $n 0}}
{{if and .CanCompareOrPull .IsViewBranch (not .Repository.IsArchived)}}