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    use std::env;

    use crate::error::Result;
    use crate::map::Map;
    use crate::source::Source;
    use crate::value::{Value, ValueKind};

    #[must_use]
    #[derive(Clone, Debug, Default)]
    pub struct Environment {
        /// Optional prefix that will limit access to the environment to only keys that
        /// begin with the defined prefix.
        ///
        /// A prefix with a separator of `_` is tested to be present on each key before its considered
        /// to be part of the source environment.
        ///
        /// For example, the key `CONFIG_DEBUG` would become `DEBUG` with a prefix of `config`.
        prefix: Option<String>,

        /// Optional character sequence that separates the prefix from the rest of the key
        prefix_separator: Option<String>,

        /// Optional character sequence that separates each key segment in an environment key pattern.
        /// Consider a nested configuration such as `redis.password`, a separator of `_` would allow
        /// an environment key of `REDIS_PASSWORD` to match.
        separator: Option<String>,

        /// Optional character sequence that separates each env value into a vector. only works when try_parsing is set to true
        /// Once set, you cannot have type String on the same environment, unless you set list_parse_keys.
        list_separator: Option<String>,
        /// A list of keys which should always be parsed as a list. If not set you can have only Vec<String> or String (not both) in one environment.
        list_parse_keys: Option<Vec<String>>,

        /// Ignore empty env values (treat as unset).
        ignore_empty: bool,

        /// Parses booleans, integers and floats if they're detected (can be safely parsed).
        try_parsing: bool,

        // Preserve the prefix while parsing
        keep_prefix: bool,

        /// Alternate source for the environment. This can be used when you want to test your own code
        /// using this source, without the need to change the actual system environment variables.
        ///
        /// ## Example
        ///
        /// ```rust
        /// # use config::{Environment, Config};
        /// # use serde::Deserialize;
        /// # use std::collections::HashMap;
        /// # use std::convert::TryInto;
        /// #
        /// #[test]
        /// fn test_config() -> Result<(), config::ConfigError> {
        ///   #[derive(Clone, Debug, Deserialize)]
        ///   struct MyConfig {
        ///     pub my_string: String,
        ///   }
        ///
        ///   let source = Environment::default()
        ///     .source(Some({
        ///       let mut env = HashMap::new();
        ///       env.insert("MY_STRING".into(), "my-value".into());
        ///       env
        ///   }));
        ///
        ///   let config: MyConfig = Config::builder()
        ///     .add_source(source)
        ///     .build()?
        ///     .try_into()?;
        ///   assert_eq!(config.my_string, "my-value");
        ///
        ///   Ok(())
        /// }
        /// ```
        source: Option<Map<String, String>>,
    }

    impl Environment {
        #[deprecated(since = "0.12.0", note = "please use 'Environment::default' instead")]
        pub fn new() -> Self {
            Self::default()
        }

        pub fn with_prefix(s: &str) -> Self {
            Self {
                prefix: Some(s.into()),
                ..Self::default()
            }
        }

        pub fn prefix(mut self, s: &str) -> Self {
            self.prefix = Some(s.into());
            self
        }

        pub fn prefix_separator(mut self, s: &str) -> Self {
            self.prefix_separator = Some(s.into());
            self
        }

        pub fn separator(mut self, s: &str) -> Self {
            self.separator = Some(s.into());
            self
        }

        /// When set and try_parsing is true, then all environment variables will be parsed as [`Vec<String>`] instead of [`String`].
        /// See [`with_list_parse_key`] when you want to use [`Vec<String>`] in combination with [`String`].
        pub fn list_separator(mut self, s: &str) -> Self {
            self.list_separator = Some(s.into());
            self
        }

        /// Add a key which should be parsed as a list when collecting [`Value`]s from the environment.
        /// Once list_separator is set, the type for string is [`Vec<String>`].
        /// To switch the default type back to type Strings you need to provide the keys which should be [`Vec<String>`] using this function.
        pub fn with_list_parse_key(mut self, key: &str) -> Self {
            if self.list_parse_keys == None {
                self.list_parse_keys = Some(vec![key.into()])
            } else {
                self.list_parse_keys = self.list_parse_keys.map(|mut keys| {
                    keys.push(key.into());
                    keys
                });
            }
            self
        }

        pub fn ignore_empty(mut self, ignore: bool) -> Self {
            self.ignore_empty = ignore;
            self
        }

        /// Note: enabling `try_parsing` can reduce performance it will try and parse
        /// each environment variable 3 times (bool, i64, f64)
        pub fn try_parsing(mut self, try_parsing: bool) -> Self {
            self.try_parsing = try_parsing;
            self
        }

        pub fn keep_prefix(mut self, keep: bool) -> Self {
            self.keep_prefix = keep;
            self
        }

        pub fn source(mut self, source: Option<Map<String, String>>) -> Self {
            self.source = source;
            self
        }
    }

    impl Source for Environment {
        fn clone_into_box(&self) -> Box<dyn Source + Send + Sync> {
            Box::new((*self).clone())
        }

        fn collect(&self) -> Result<Map<String, Value>> {
            let mut m = Map::new();
            let uri: String = "the environment".into();

            let separator = self.separator.as_deref().unwrap_or("");
            let prefix_separator = match (self.prefix_separator.as_deref(), self.separator.as_deref()) {
                (Some(pre), _) => pre,
                (None, Some(sep)) => sep,
                (None, None) => "_",
            };

            // Define a prefix pattern to test and exclude from keys
            let prefix_pattern = self
                .prefix
                .as_ref()
                .map(|prefix| format!("{}{}", prefix, prefix_separator).to_lowercase());

            let collector = |(key, value): (String, String)| {
                // Treat empty environment variables as unset
                if self.ignore_empty && value.is_empty() {
                    return;
                }

                let mut key = key.to_lowercase();

                // Check for prefix
                if let Some(ref prefix_pattern) = prefix_pattern {
                    if key.starts_with(prefix_pattern) {
                        if !self.keep_prefix {
                            // Remove this prefix from the key
                            key = key[prefix_pattern.len()..].to_string();
                        }
                    } else {
                        // Skip this key
                        return;
                    }
                }

                // If separator is given replace with `.`
                if !separator.is_empty() {
                    key = key.replace(separator, ".");
                }

                let value = if self.try_parsing {
                    // convert to lowercase because bool parsing expects all lowercase
                    if let Ok(parsed) = value.to_lowercase().parse::<bool>() {
                        ValueKind::Boolean(parsed)
                    } else if let Ok(parsed) = value.parse::<i64>() {
                        ValueKind::I64(parsed)
                    } else if let Ok(parsed) = value.parse::<f64>() {
                        ValueKind::Float(parsed)
                    } else if let Some(separator) = &self.list_separator {
                        if let Some(keys) = &self.list_parse_keys {
                            if keys.contains(&key) {
                                let v: Vec<Value> = value
                                    .split(separator)
                                    .map(|s| Value::new(Some(&uri), ValueKind::String(s.to_string())))
                                    .collect();
                                ValueKind::Array(v)
                            } else {
                                ValueKind::String(value)
                            }
                        } else {
                            let v: Vec<Value> = value
                                .split(separator)
                                .map(|s| Value::new(Some(&uri), ValueKind::String(s.to_string())))
                                .collect();
                            ValueKind::Array(v)
                        }
                    } else {
                        ValueKind::String(value)
                    }
                } else {
                    ValueKind::String(value)
                };

                m.insert(key, Value::new(Some(&uri), value));
            };

            match &self.source {
                Some(source) => source.clone().into_iter().for_each(collector),
                None => env::vars().for_each(collector),
            }

            Ok(m)
        }
    }