statime-config 0.1.0

use std::{collections::HashMap, path::PathBuf};

use crate::{ConfigError, tree::path::ConfigPath};

/// Whether to merge by overriding values, or by rejecting any overlapping values.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum MergePolicy {
    /// Reject any overlapping values and return an error if any is found.
    RejectOverlap,
    /// Override any overlapping values with the incoming value.
    Override,
}

/// Where a configuration file/value is coming from.
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub enum Origin {
    /// The source is the main configuration file.
    MainConfig(PathBuf),
    /// The source is a system configuration file.
    SystemConfig(PathBuf),
    /// The source is a built-in default value.
    BuiltInDefault,
}

/// A unique identifier that represents an origin.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct OriginId(usize);

impl OriginId {
    /// The reserved id for [`Origin::BuiltInDefault`].
    pub const BUILT_IN_DEFAULT: Self = Self(0);
}

/// Tracks the origin ids to specific origins.
#[derive(Debug)]
pub struct ProvenanceTracker {
    origins: HashMap<OriginId, Origin>,
}

impl ProvenanceTracker {
    /// Create an empty provenance tracker.
    pub fn new() -> Self {
        Self {
            origins: HashMap::new(),
        }
    }

    /// Track an origin, returns the origin id for that origin.
    pub fn track(&mut self, origin: Origin) -> OriginId {
        if origin == Origin::BuiltInDefault {
            return OriginId::BUILT_IN_DEFAULT;
        }
        if let Some((id, _)) = self.origins.iter().find(|(_, o)| *o == &origin) {
            return *id;
        }
        // ids start at one, since zero is reserved for the built-in default
        let id = OriginId(self.origins.len() + 1);
        self.origins.insert(id, origin);
        id
    }

    /// Get the origin for a given origin id, if it is known.
    pub fn get_origin(&self, id: OriginId) -> Option<&Origin> {
        if id == OriginId::BUILT_IN_DEFAULT {
            return Some(&Origin::BuiltInDefault);
        }
        self.origins.get(&id)
    }
}

impl Default for ProvenanceTracker {
    fn default() -> Self {
        Self::new()
    }
}

/// The context within the current merge operation. Generated code only ever
/// walks it with [`MergeContext::at`]; the rest is the loader's business.
pub struct MergeContext<'a> {
    #[doc(hidden)]
    pub policy: MergePolicy,
    pub path: ConfigPath,
    provenance: &'a ProvenanceTracker,
}

impl<'a> MergeContext<'a> {
    pub(crate) fn new(policy: MergePolicy, provenance: &'a ProvenanceTracker) -> Self {
        Self {
            policy,
            path: ConfigPath::root(),
            provenance,
        }
    }

    /// Run `f` with `field` appended to the path being merged.
    pub fn at<R>(&mut self, field: &'static str, f: impl FnOnce(&mut Self) -> R) -> R {
        self.path.push_field(field);
        let result = f(self);
        self.path.pop();
        result
    }

    /// The document an id stands for, for use in a diagnostic.
    pub(crate) fn origin(&self, id: Option<OriginId>) -> Option<Origin> {
        id.and_then(|id| self.provenance.get_origin(id)).cloned()
    }
}

/// Allows the merging of two values following the merge policy in the merge
/// context.
pub trait Merge {
    fn merge(&mut self, incoming: Self, context: &mut MergeContext<'_>) -> Result<(), ConfigError>;
}

#[cfg(test)]
mod tests {
    use crate::{
        Setting,
        fixture::{PartialPool, PartialServer, PartialSource},
    };

    use super::*;

    #[test]
    fn the_built_in_default_origin_is_reserved() {
        let mut tracker = ProvenanceTracker::new();

        // it resolves without ever having been registered
        assert_eq!(
            tracker.get_origin(OriginId::BUILT_IN_DEFAULT),
            Some(&Origin::BuiltInDefault)
        );
        // and registering it hands back the reserved id rather than a new one
        assert_eq!(
            tracker.track(Origin::BuiltInDefault),
            OriginId::BUILT_IN_DEFAULT
        );

        // no tracked origin may collide with the reserved id
        let first = tracker.track(Origin::MainConfig("/etc/ntp.toml".into()));
        let second = tracker.track(Origin::SystemConfig("/etc/ntp.d/a.toml".into()));
        assert_ne!(first, OriginId::BUILT_IN_DEFAULT);
        assert_ne!(second, OriginId::BUILT_IN_DEFAULT);
        assert_ne!(first, second);
        assert_eq!(
            tracker.get_origin(first),
            Some(&Origin::MainConfig("/etc/ntp.toml".into()))
        );
    }

    #[test]
    fn merge_of_enum_merges_identical() {
        let mut left = PartialSource::Server(PartialServer {
            address: Setting::Unset,
            version: Setting::Set {
                value: 5,
                origin: None,
            },
        });

        let right = PartialSource::Server(PartialServer {
            address: Setting::Set {
                value: "hello".into(),
                origin: None,
            },
            version: Setting::Unset,
        });

        left.merge(
            right,
            &mut MergeContext {
                policy: MergePolicy::RejectOverlap,
                path: ConfigPath::root(),
                provenance: &ProvenanceTracker::new(),
            },
        )
        .unwrap();
        assert_eq!(
            left,
            PartialSource::Server(PartialServer {
                address: Setting::Set {
                    value: "hello".into(),
                    origin: None,
                },
                version: Setting::Set {
                    value: 5,
                    origin: None,
                },
            })
        );

        let third = PartialSource::Server(PartialServer {
            address: Setting::Set {
                value: "bla".into(),
                origin: None,
            },
            version: Setting::Unset,
        });

        left.merge(
            third,
            &mut MergeContext {
                policy: MergePolicy::Override,
                path: ConfigPath::root(),
                provenance: &ProvenanceTracker::new(),
            },
        )
        .unwrap();
        assert_eq!(
            left,
            PartialSource::Server(PartialServer {
                address: Setting::Set {
                    value: "bla".into(),
                    origin: None,
                },
                version: Setting::Set {
                    value: 5,
                    origin: None,
                },
            })
        );
    }

    #[test]
    fn merge_of_enum_overrides_when_allowed() {
        let mut left = PartialSource::Server(PartialServer {
            address: Setting::Unset,
            version: Setting::Set {
                value: 5,
                origin: None,
            },
        });

        let right = PartialSource::Pool(PartialPool {
            address: Setting::Set {
                value: "hello".into(),
                origin: None,
            },
            count: Setting::Unset,
        });

        left.merge(
            right,
            &mut MergeContext {
                policy: MergePolicy::Override,
                path: ConfigPath::root(),
                provenance: &ProvenanceTracker::new(),
            },
        )
        .unwrap();

        assert_eq!(
            left,
            PartialSource::Pool(PartialPool {
                address: Setting::Set {
                    value: "hello".into(),
                    origin: None,
                },
                count: Setting::Unset,
            })
        );
    }

    #[test]
    fn merge_of_enum_rejects_different_variants_when_not_overriding() {
        let mut left = PartialSource::Server(PartialServer {
            address: Setting::Unset,
            version: Setting::Set {
                value: 5,
                origin: None,
            },
        });

        let right = PartialSource::Pool(PartialPool {
            address: Setting::Set {
                value: "hello".into(),
                origin: None,
            },
            count: Setting::Unset,
        });

        assert!(matches!(
            left.merge(
                right,
                &mut MergeContext {
                    policy: MergePolicy::RejectOverlap,
                    path: ConfigPath::root(),
                    provenance: &ProvenanceTracker::new()
                }
            ),
            Err(ConfigError::MismatchedVariants {
                current: "server",
                incoming: "Pool",
                ..
            })
        ));
    }
}