export default function collapseDuplicateDeclarations() {
return (root) => {
root.walkRules((node) => {
let seen = new Map()
let droppable = new Set([])
let byProperty = new Map()
node.walkDecls((decl) => {
// This could happen if we have nested selectors. In that case the
// parent will loop over all its declarations but also the declarations
// of nested rules. With this we ensure that we are shallowly checking
// declarations.
if (decl.parent !== node) {
return
}
if (seen.has(decl.prop)) {
// Exact same value as what we have seen so far
if (seen.get(decl.prop).value === decl.value) {
// Keep the last one, drop the one we've seen so far
droppable.add(seen.get(decl.prop))
// Override the existing one with the new value. This is necessary
// so that if we happen to have more than one declaration with the
// same value, that we keep removing the previous one. Otherwise we
// will only remove the *first* one.
seen.set(decl.prop, decl)
return
}
// Not the same value, so we need to check if we can merge it so
// let's collect it first.
if (!byProperty.has(decl.prop)) {
byProperty.set(decl.prop, new Set())
}
byProperty.get(decl.prop).add(seen.get(decl.prop))
byProperty.get(decl.prop).add(decl)
}
seen.set(decl.prop, decl)
})
// Drop all the duplicate declarations with the exact same value we've
// already seen so far.
for (let decl of droppable) {
decl.remove()
}
// Analyze the declarations based on its unit, drop all the declarations
// with the same unit but the last one in the list.
for (let declarations of byProperty.values()) {
let byUnit = new Map()
for (let decl of declarations) {
let unit = resolveUnit(decl.value)
if (unit === null) {
// We don't have a unit, so should never try and collapse this
// value. This is because we can't know how to do it in a correct
// way (e.g.: overrides for older browsers)
continue
}
if (!byUnit.has(unit)) {
byUnit.set(unit, new Set())
}
byUnit.get(unit).add(decl)
}
for (let declarations of byUnit.values()) {
// Get all but the last one
let removableDeclarations = Array.from(declarations).slice(0, -1)
for (let decl of removableDeclarations) {
decl.remove()
}
}
}
})
}
}
let UNITLESS_NUMBER = Symbol('unitless-number')
function resolveUnit(input) {
let result = /^-?\d*.?\d+([\w%]+)?$/g.exec(input)
if (result) {
return result[1] ?? UNITLESS_NUMBER
}
return null
}