const postcss = require('postcss')

const defaults = {

rootSelector: ':root',

typeRatio: 1.2,

ratioProperty: '--type-ratio'

}

module.exports = postcss.plugin('type-scale', (opts = defaults) => {

const options = Object.assign(defaults, opts)

return css => {

const typeRatio = getTypeRatio(css, options)

css.walkDecls('font-size', decl => {

// Replace only if it's a unitless value

if (/\d+$/.test(decl.value)) decl.value = getSize(decl.value, typeRatio)

})

}

})

function getTypeRatio (css, opts) {

// Start with the default ratio

let typeRatio = opts.typeRatio

// Walk over all the root selectors

css.walkRules(opts.rootSelector, rule => {

// Omit the process if the selector is inside a print media query

if (rule.parent && rule.parent.params === 'print') return

// Walk over all the font-size rules

rule.walkDecls(opts.ratioProperty, decl => {

typeRatio = parseFloat(decl.value)

})

})

return typeRatio

}

function getSize(val, ratio) {

return parseFloat(Math.pow(ratio, parseInt(val) - 2).toFixed(4)) + 'rem'

}