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
}