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Machinery for configuring Http4k apps in a typesafe way

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package org.http4k.config

import org.http4k.format.JacksonYaml
import java.io.File
import java.io.FileNotFoundException
import java.io.InputStreamReader
import java.util.Properties

/**
 * Read a YAML file into environments, prepending all of the nested levels into the property names
 */
fun Environment.Companion.fromYaml(file: File): Environment {
    val map = JacksonYaml.asA>(file.reader().use(InputStreamReader::readText))

    return MapEnvironment.from(map.flatten().toMap().toProperties())
}

/**
 * Read a YAML file from resources into environments, prepending all of the nested levels into the property names
 */
fun Environment.Companion.fromYaml(resource: String): Environment {
    val map = Environment.Companion::class.java.getResourceAsStream("/${resource.removePrefix("/")}")
        ?.let { input -> JacksonYaml.asA>(input.reader().use(InputStreamReader::readText)) }
        ?: throw FileNotFoundException(resource)

    return MapEnvironment.from(map.flatten().toMap().toProperties())
}

private fun Map<*, *>.flatten(): List> {
    fun convert(key: Any?, value: Any?): List> {
        val keyString = (key ?: "").toString()

        return when (value) {
            is List<*> -> listOf(keyString to value.flatMap { convert(null, it) }.joinToString(",") { it.second })
            is Map<*, *> -> value.flatten().map { "$keyString.${it.first}" to it.second }
            else -> listOf((keyString to (value ?: "").toString()))
        }
    }

    return entries.fold(listOf()) { acc, (key, value) -> acc + convert(key, value) }
}

fun Environment.Companion.fromConfigFile(file: File): Environment {
    val (_, named) = file.reader().readLines().foldRight(emptyList() to mapOf()) { next, (running, done) ->
        if (next.startsWith("[")) {
            val key = next.trim('[', ']').replace(' ', '.')
            emptyList() to done + Properties().apply { load(running.joinToString("\n").reader()) }.toMap().map { key + "." + it.key to it.value.toString() }.toMap()
        } else {
            (running + next) to done
        }
    }
    return MapEnvironment.from(named.toProperties())
}