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.

9 files changed, 20 insertions, 19 deletions

diff --git a/templates/repo/branch/list.tmpl b/templates/repo/branch/list.tmpl
index 162fa18764..6dd6aecb37 100644
--- a/templates/repo/branch/list.tmpl
+++ b/templates/repo/branch/list.tmpl
@@ -69,11 +69,12 @@
 										<div class="commit-divergence">
 											<div class="bar-group">
 												<div class="count count-behind">{{.CommitsBehind}}</div>
-												<div class="bar bar-behind" style="width: {{percentage .CommitsBehind .CommitsBehind .CommitsAhead}}%"></div>
+												{{/* old code bears 0/0.0 = NaN output, so it might output invalid "width: NaNpx", it just works and doesn't caues any problem. */}}
+												<div class="bar bar-behind" style="width: {{Eval 100 "*" .CommitsBehind "/" "(" .CommitsBehind "+" .CommitsAhead "+" 0.0 ")"}}%"></div>
 											</div>
 											<div class="bar-group">
 												<div class="count count-ahead">{{.CommitsAhead}}</div>
-												<div class="bar bar-ahead" style="width: {{percentage .CommitsAhead .CommitsBehind .CommitsAhead}}%"></div>
+												<div class="bar bar-ahead" style="width: {{Eval 100 "*" .CommitsAhead "/" "(" .CommitsBehind "+" .CommitsAhead "+" 0.0 ")"}}%"></div>
 											</div>
 										</div>
 										{{end}}
diff --git a/templates/repo/commits_list_small.tmpl b/templates/repo/commits_list_small.tmpl
index 40a9843c82..5bc159de81 100644
--- a/templates/repo/commits_list_small.tmpl
+++ b/templates/repo/commits_list_small.tmpl
@@ -2,7 +2,7 @@
 <div class="timeline-item commits-list">
 {{range .comment.Commits}}
 	{{$tag := printf "%s-%d" $.comment.HashTag $index}}
-	{{$index = Add $index 1}}
+	{{$index = Eval $index "+" 1}}
 	<div class="singular-commit" id="{{$tag}}">
 		<span class="badge badge-commit">{{svg "octicon-git-commit"}}</span>
 		{{if .User}}
diff --git a/templates/repo/diff/stats.tmpl b/templates/repo/diff/stats.tmpl
index 1e62fa6d6a..1e466d6b46 100644
--- a/templates/repo/diff/stats.tmpl
+++ b/templates/repo/diff/stats.tmpl
@@ -1,4 +1,4 @@
-{{Add .file.Addition .file.Deletion}}
-<span class="diff-stats-bar gt-mx-3" data-tooltip-content="{{.root.locale.Tr "repo.diff.stats_desc_file" (Add .file.Addition .file.Deletion) .file.Addition .file.Deletion | Str2html}}">
+{{Eval .file.Addition "+" .file.Deletion}}
+<span class="diff-stats-bar gt-mx-3" data-tooltip-content="{{.root.locale.Tr "repo.diff.stats_desc_file" (Eval .file.Addition "+" .file.Deletion) .file.Addition .file.Deletion | Str2html}}">
 	<div class="diff-stats-add-bar" style="width: {{DiffStatsWidth .file.Addition .file.Deletion}}%"></div>
 </span>
diff --git a/templates/repo/editor/edit.tmpl b/templates/repo/editor/edit.tmpl
index 6ecc5e1460..0ac6e93d49 100644
--- a/templates/repo/editor/edit.tmpl
+++ b/templates/repo/editor/edit.tmpl
@@ -12,7 +12,7 @@
 					<div class="ui breadcrumb field {{if .Err_TreePath}}error{{end}}">
 						<a class="section" href="{{$.BranchLink}}">{{.Repository.Name}}</a>
 						{{$n := len .TreeNames}}
-						{{$l := Subtract $n 1}}
+						{{$l := Eval $n "-" 1}}
 						{{range $i, $v := .TreeNames}}
 							<div class="divider"> / </div>
 							{{if eq $i $l}}
diff --git a/templates/repo/editor/upload.tmpl b/templates/repo/editor/upload.tmpl
index 426d53db7a..88cdf61c74 100644
--- a/templates/repo/editor/upload.tmpl
+++ b/templates/repo/editor/upload.tmpl
@@ -10,7 +10,7 @@
 					<div class="ui breadcrumb field {{if .Err_TreePath}}error{{end}}">
 						<a class="section" href="{{$.BranchLink}}">{{.Repository.Name}}</a>
 						{{$n := len .TreeNames}}
-						{{$l := Subtract $n 1}}
+						{{$l := Eval $n "-" 1}}
 						{{range $i, $v := .TreeNames}}
 							<div class="divider"> / </div>
 							{{if eq $i $l}}
diff --git a/templates/repo/graph/svgcontainer.tmpl b/templates/repo/graph/svgcontainer.tmpl
index 26528ef81a..99c3c87399 100644
--- a/templates/repo/graph/svgcontainer.tmpl
+++ b/templates/repo/graph/svgcontainer.tmpl
@@ -1,22 +1,22 @@
 <div id="rel-container">
-	<svg viewbox="{{Mul .Graph.MinColumn 5}} {{Mul .Graph.MinRow 12}} {{Add (Mul .Graph.Width 5) 5}} {{Mul .Graph.Height 12}}" width="{{Add (Mul .Graph.Width 10) 10}}px">
+	<svg viewbox="{{Eval .Graph.MinColumn "*" 5}} {{Eval .Graph.MinRow "*" 12}} {{Eval .Graph.Width "*" 5 "+" 5}} {{Eval .Graph.Height "*" 12}}" width="{{Eval .Graph.Width "*" 10 "+" 10}}px">
 		{{range $flowid, $flow := .Graph.Flows}}
 			<g id="flow-{{$flow.ID}}" class="flow-group flow-color-{{$flow.ColorNumber}} flow-color-16-{{$flow.Color16}}" data-flow="{{$flow.ID}}" data-color="{{$flow.ColorNumber}}">
 				<path d="{{range $i, $glyph := $flow.Glyphs -}}
 					{{- if or (eq $glyph.Glyph '*') (eq $glyph.Glyph '|') -}}
-						M {{Add (Mul $glyph.Column 5) 5}} {{Add (Mul $glyph.Row 12) 0}} v 12 {{/* */ -}}
+						M {{Eval $glyph.Column "*" 5 "+" 5}} {{Eval $glyph.Row "*" 12 "+" 0}} v 12 {{/* */ -}}
 					{{- else if eq $glyph.Glyph '/' -}}
-						M {{Add (Mul $glyph.Column 5) 10}} {{Add (Mul $glyph.Row 12) 0}} l -10 12 {{/* */ -}}
+						M {{Eval $glyph.Column "*" 5 "+" 10}} {{Eval $glyph.Row "*" 12 "+" 0}} l -10 12 {{/* */ -}}
 					{{- else if eq $glyph.Glyph '\\' -}}
-						M {{Add (Mul $glyph.Column 5) 0}} {{Add (Mul $glyph.Row 12) 0}} l 10 12 {{/* */ -}}
+						M {{Eval $glyph.Column "*" 5 "+" 0}} {{Eval $glyph.Row "*" 12 "+" 0}} l 10 12 {{/* */ -}}
 					{{- else if or (eq $glyph.Glyph '-') (eq $glyph.Glyph '.') -}}
-						M {{Add (Mul $glyph.Column 5) 0}} {{Add (Mul $glyph.Row 12) 12}} h 5 {{/* */ -}}
+						M {{Eval $glyph.Column "*" 5 "+" 0}} {{Eval $glyph.Row "*" 12 "+" 12}} h 5 {{/* */ -}}
 					{{- else if eq $glyph.Glyph '_' -}}
-						M {{Add (Mul $glyph.Column 5) 0}} {{Add (Mul $glyph.Row 12) 12}} h 10 {{/* */ -}}
+						M {{Eval $glyph.Column "*" 5 "+" 0}} {{Eval $glyph.Row "*" 12 "+" 12}} h 10 {{/* */ -}}
 					{{- end -}}
 				{{- end}}" stroke-width="1" fill="none" id="flow-{{$flow.ID}}-path" stroke-linecap="round"></path>
 				{{range $flow.Commits}}
-					<circle class="flow-commit" cx="{{Add (Mul .Column 5) 5}}" cy="{{Add (Mul .Row 12) 6}}" r="2.5" stroke="none" id="flow-commit-{{.Rev}}" data-rev="{{.Rev}}"></circle>
+					<circle class="flow-commit" cx="{{Eval .Column "*" 5 "+" 5}}" cy="{{Eval .Row "*" 12 "+" 6}}" r="2.5" stroke="none" id="flow-commit-{{.Rev}}" data-rev="{{.Rev}}"></circle>
 				{{end}}
 			</g>
 		{{end}}
diff --git a/templates/repo/home.tmpl b/templates/repo/home.tmpl
index e544b7aab5..8de54611c3 100644
--- a/templates/repo/home.tmpl
+++ b/templates/repo/home.tmpl
@@ -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)}}
diff --git a/templates/repo/view_file.tmpl b/templates/repo/view_file.tmpl
index 36b50e0c7f..7ee08c9efc 100644
--- a/templates/repo/view_file.tmpl
+++ b/templates/repo/view_file.tmpl
@@ -100,7 +100,7 @@
 				<table>
 					<tbody>
 						{{range $idx, $code := .FileContent}}
-						{{$line := Add $idx 1}}
+						{{$line := Eval $idx "+" 1}}
 						<tr>
 							<td id="L{{$line}}" class="lines-num"><span id="L{{$line}}" data-line-number="{{$line}}"></span></td>
 							{{if $.EscapeStatus.Escaped}}
diff --git a/templates/repo/wiki/view.tmpl b/templates/repo/wiki/view.tmpl
index 15553cedd8..1aa21b906a 100644
--- a/templates/repo/wiki/view.tmpl
+++ b/templates/repo/wiki/view.tmpl
@@ -80,8 +80,8 @@
 							</summary>
 							{{$level := 0}}
 							{{range .toc}}
-								{{if lt $level .Level}}{{range Iterate (Subtract .Level $level)}}<ul>{{end}}{{end}}
-								{{if gt $level .Level}}{{range Iterate (Subtract $level .Level)}}</ul>{{end}}{{end}}
+								{{if lt $level .Level}}{{range Iterate (Eval .Level "-" $level)}}<ul>{{end}}{{end}}
+								{{if gt $level .Level}}{{range Iterate (Eval $level "-" .Level)}}</ul>{{end}}{{end}}
 								{{$level = .Level}}
 								<li><a href="#{{.ID}}">{{.Text}}</a></li>
 							{{end}}