Optimize for large reports

[mlarraz]

Problem

A real-world coverage report with 1705 source files produces a 41MB
HTML file (~1.9M lines) that causes significant browser slowdown:

• 96% of the file is the source files section (~41.3MB)
• 51% of that section is whitespace from ERB template formatting (~21MB)
• 142,254 <li> elements rendered into the DOM at page load (hidden)
• 142K event handlers individually bound to line number elements
• An unnecessary <div> wrapper around each <li> adds 142K extra DOM nodes

Changes

Phase 1: Template Optimizations

ERB whitespace trimming (lib/simplecov-html.rb, all .erb views)

• Add trim_mode: '-' to the ERB constructor
• Add <%- / -%> trim markers to control flow tags in all templates
• Eliminates ~21MB of blank lines produced by ERB conditionals/loops

Remove <div> wrapper around <li> (views/source_file.erb)

• These were semantically invalid inside <ol> and unnecessary
• Saves ~2.8MB and 142K DOM nodes

Conditional data-hits attribute (views/source_file.erb)

• Only emit data-hits="N" when line.coverage is truthy
• Previously emitted data-hits="" for ~83K "never" lines with no
  JS/CSS referencing the empty attribute

Compact covered_percent.erb

• Single-line template avoids multi-line whitespace across ~3,400 calls

Phase 2: Browser Performance

<template> tags for source files (views/layout.erb)

• Each source file is wrapped in <template id="tmpl-SHA1">
• <template> content is parsed but NOT rendered into the DOM until
  explicitly activated — removes ~500K DOM nodes from initial page load

Template materialization on demand (assets/javascripts/application.js)

• New materializeSourceFile() function clones template content into
  the .source_files container when a file is first viewed
• Syntax highlighting applied on materialization (same deferred approach)
• All code paths updated: click, colorbox onLoad, popstate, deep links

Event delegation for line number clicks (assets/javascripts/application.js)

• Replaced direct binding on 142K elements with a single delegated
  event handler on document
• Required for <template> approach and eliminates 142K event bindings

Results

Tested against a real coverage report with 1705 source files:

Metric	Before	After	Change
File size	41 MB	25 MB	-39%
Lines	1,944,847	573,916	-70%
Blank lines	1,090,596	3,578	-99.7%
<div> wrappers	142,255	2	-142,253
data-hits attrs	142,253	59,101	-83,152
DOM nodes at load	~500K+	~few K	deferred via <template>
Event bindings	142K	1	delegated

The remaining 25MB is predominantly the actual source code content
inside <li> elements, which is irreducible.

Files Modified

File	Changes
lib/simplecov-html.rb	trim_mode: '-' on ERB constructor
views/source_file.erb	Trim markers, remove <div>, conditional data-hits
views/layout.erb	Trim markers, <template> tags around source files
views/file_list.erb	Trim markers on control flow tags
views/covered_percent.erb	Single-line template
assets/javascripts/application.js	Template materialization, event delegation
public/application.js	Re-compiled asset

Test Plan

• bundle exec rake test passes
• Generate a coverage report against a test project and compare HTML file size
• Open in browser: file list loads, clicking a file shows source in modal
• Line numbers are clickable and scroll to correct position
• Syntax highlighting works on opened files
• Back/forward navigation works correctly
• Deep-linking to a specific file/line via URL hash works