//! Units of measure for rate × quantity lines. Mirrors src/lib/units.ts: the ids below are stored in
//! invoices and presets, and any other text that passes `validate` is a custom unit stored as typed.

/// Built-in unit ids with the ERPNext UOM each one maps to by default. Whole-number UOMs ("Nos", "Set")
/// reject fractional quantities, so units that are usually fractional map to UOMs that allow them.
pub const KNOWN: &[(&str, &str)] = &[
    ("unit", "Nos"),
    ("piece", "Nos"),
    ("set", "Set"),
    ("session", "Nos"),
    ("second", "Second"),
    ("minute", "Minute"),
    ("hour", "Hour"),
    ("day", "Day"),
    ("week", "Week"),
    ("month", "Month"),
    ("km", "Kilometer"),
    ("m", "Meter"),
    ("sqft", "Square Foot"),
    ("sqm", "Square Meter"),
    ("kg", "Kg"),
    ("litre", "Litre"),
];

/// Longest custom unit; it lands in a measured PDF column.
pub const MAX_CUSTOM_LEN: usize = 12;

pub fn is_known(unit: &str) -> bool {
    KNOWN.iter().any(|(id, _)| *id == unit)
}

/// The ERPNext UOM that suits a built-in unit.
pub fn default_uom(unit: &str) -> Option<&'static str> {
    KNOWN
        .iter()
        .find(|(id, _)| *id == unit)
        .map(|(_, uom)| *uom)
}

fn custom_problem(text: &str) -> Option<String> {
    let t = text.trim();
    if t.is_empty() {
        return Some("a unit is required".into());
    }
    if t.chars().count() > MAX_CUSTOM_LEN {
        return Some(format!(
            "a custom unit has at most {MAX_CUSTOM_LEN} characters"
        ));
    }
    let mut chars = t.chars();
    let first_ok = chars.next().is_some_and(|c| c.is_ascii_alphanumeric());
    if !first_ok || !chars.all(|c| c.is_ascii_alphanumeric() || matches!(c, ' ' | '.' | '/' | '-'))
    {
        return Some("a unit may use letters, digits, spaces, . / and - only".into());
    }
    None
}

/// The unit as it is stored: trimmed, and a built-in id in lower case. An empty value is the generic unit.
pub fn normalize(unit: &str) -> String {
    let t = unit.trim();
    if t.is_empty() {
        return "unit".to_string();
    }
    let lower = t.to_ascii_lowercase();
    if is_known(&lower) {
        lower
    } else {
        t.to_string()
    }
}

/// Checks a unit that is about to be stored. Errors read as `Unit "x": <problem>`.
pub fn validate(unit: &str) -> Result<(), String> {
    if unit.trim().is_empty() {
        return Err("A unit is required".to_string());
    }
    if is_known(&normalize(unit)) {
        return Ok(());
    }
    match custom_problem(unit) {
        None => Ok(()),
        Some(p) => Err(format!("Unit \"{}\": {p}", unit.trim())),
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn known_units_are_accepted_in_any_case() {
        for (id, _) in KNOWN {
            assert!(validate(id).is_ok(), "{id}");
        }
        assert_eq!(normalize(" HOUR "), "hour");
        assert_eq!(normalize(""), "unit");
    }

    #[test]
    fn custom_units_follow_the_same_rules_as_the_ui() {
        assert!(validate("bundle").is_ok());
        assert!(validate("sq.ft/day").is_ok());
        assert_eq!(normalize("  bundle "), "bundle");
        assert!(validate("a".repeat(13).as_str()).is_err());
        assert!(validate("₹").is_err());
        assert!(validate("-x").is_err());
        assert!(validate("").is_err());
        assert!(validate("   ").is_err());
        assert!(validate("bad;drop").is_err());
    }

    #[test]
    fn the_original_five_ids_are_unchanged() {
        for id in ["second", "minute", "hour", "session", "unit"] {
            assert!(is_known(id), "{id}");
        }
    }

    #[test]
    fn fractional_units_do_not_map_to_whole_number_uoms() {
        for id in [
            "second", "minute", "hour", "day", "week", "month", "km", "m", "sqft", "sqm", "kg",
            "litre",
        ] {
            assert_ne!(default_uom(id), Some("Nos"), "{id}");
        }
    }
}