// common/src/system_column.rs
//! The columns the server puts on every managed table.
//!
//! They are the one exception to the rule that a physical column name never
//! leaves the server. A user column is stored under its ordinal and shown only
//! under its alias, so an unmapped name at the public boundary is a leak; these
//! have no alias to hide behind, because the server -- not the user -- chose
//! their names. The list lives here once and the `CREATE TABLE` fragments that
//! make it true live next to it. Alias validation may still be conditional: the
//! virtual `account` API name conflicts only on ACCOUNTING-enabled tables.

/// A column every managed table carries, named the same in Postgres and in the
/// public API.
pub struct SystemColumn {
    pub name: &'static str,
    /// The `CREATE TABLE` fragment that declares it.
    pub definition: &'static str,
}

/// Declared ahead of the user columns.
pub const LEADING_SYSTEM_COLUMNS: [SystemColumn; 3] = [
    SystemColumn {
        name: "id",
        definition: "id BIGSERIAL PRIMARY KEY",
    },
    SystemColumn {
        name: "deleted",
        definition: "deleted BOOLEAN NOT NULL DEFAULT FALSE",
    },
    SystemColumn {
        name: "row_revision",
        definition: "row_revision BIGINT NOT NULL DEFAULT 1",
    },
];

/// Declared after the user columns.
pub const TRAILING_SYSTEM_COLUMNS: [SystemColumn; 1] = [SystemColumn {
    name: "created_at",
    definition: "created_at TIMESTAMPTZ DEFAULT CURRENT_TIMESTAMP",
}];

/// Present only on a table that references its profile's chart of accounts.
/// Its declaration names that profile's schema, so it is built where the
/// profile is known rather than spelled out here.
pub const ACCOUNT_REFERENCE_COLUMN: &str = "account_id";
pub const ACCOUNT_API_COLUMN: &str = "account";

/// The longest name any system column carries physically.
///
/// Anything that names a database object after a column has to reserve room
/// for the widest name it could be given, and a user column's ordinal is far
/// shorter than these. The bound is computed from the declarations above
/// rather than picked by hand, so adding a system column moves it.
///
/// [`ACCOUNT_API_COLUMN`] is deliberately absent: it is an API spelling, never
/// a physical column, so nothing is ever named after it.
pub const LONGEST_SYSTEM_COLUMN_NAME: usize = longest_system_column_name();

const fn longest_system_column_name() -> usize {
    const fn longest_of(columns: &[SystemColumn], mut longest: usize) -> usize {
        let mut index = 0;
        while index < columns.len() {
            let length = columns[index].name.len();
            if length > longest {
                longest = length;
            }
            index += 1;
        }
        longest
    }

    let longest = longest_of(&LEADING_SYSTEM_COLUMNS, ACCOUNT_REFERENCE_COLUMN.len());
    longest_of(&TRAILING_SYSTEM_COLUMNS, longest)
}

/// Every system column name, whether or not the column is on a given table.
///
/// A name is reserved for all tables even when only some tables carry the
/// column, so an alias means the same thing everywhere.
pub fn system_column_names() -> impl Iterator<Item = &'static str> {
    LEADING_SYSTEM_COLUMNS
        .iter()
        .chain(TRAILING_SYSTEM_COLUMNS.iter())
        .map(|column| column.name)
        .chain(std::iter::once(ACCOUNT_REFERENCE_COLUMN))
}

/// Whether `name` belongs to the system column vocabulary and is safe to show
/// to a client as-is. Whether a user may claim a virtual API name as an alias
/// is decided with the table's capabilities in scope.
pub fn is_system_column(name: &str) -> bool {
    name == ACCOUNT_API_COLUMN || system_column_names().any(|system_name| system_name == name)
}

/// The system column names in a message, as `'id', 'deleted', ...`.
pub fn system_column_name_list() -> String {
    system_column_names()
        .chain(std::iter::once(ACCOUNT_API_COLUMN))
        .map(|name| format!("'{name}'"))
        .collect::<Vec<_>>()
        .join(", ")
}

#[cfg(test)]
mod tests {
    use super::{
        ACCOUNT_API_COLUMN, ACCOUNT_REFERENCE_COLUMN, LONGEST_SYSTEM_COLUMN_NAME, is_system_column,
        system_column_name_list, system_column_names,
    };

    #[test]
    fn a_declared_column_is_a_system_column() {
        for name in system_column_names() {
            assert!(is_system_column(name), "{name} is declared but not public");
        }
    }

    #[test]
    fn conditional_system_names_are_recognized() {
        assert!(is_system_column(ACCOUNT_REFERENCE_COLUMN));
        assert!(is_system_column(ACCOUNT_API_COLUMN));
    }

    #[test]
    fn an_ordinal_is_not_a_system_column() {
        assert!(!is_system_column("1"));
        assert!(!is_system_column("column_1"));
    }

    /// The const derivation walks the arrays directly, so this walks the
    /// public iterator instead. A system column declared somewhere the const
    /// function does not reach shows up here as a disagreement rather than as
    /// a truncated index name much later.
    #[test]
    fn the_longest_name_bound_covers_every_system_column() {
        let longest = system_column_names()
            .map(str::len)
            .max()
            .expect("there is always at least one system column");

        assert_eq!(longest, LONGEST_SYSTEM_COLUMN_NAME);
    }

    #[test]
    fn the_name_list_reads_as_a_sentence_fragment() {
        assert_eq!(
            system_column_name_list(),
            "'id', 'deleted', 'row_revision', 'created_at', 'account_id', 'account'"
        );
    }
}