//! Reading a Splitwise export (SPEC §10.8): CSV quoting, the English or Swedish header with the member columns
//! named by full Splitwise name, and row classification (data rows, the trailing totals row, blank rows).

use std::collections::BTreeMap;

/// The fixed leading columns of the English export.
pub const ENGLISH_COLUMNS: [&str; 5] = ["Date", "Description", "Category", "Cost", "Currency"];
/// The fixed leading columns of the Swedish export.
pub const SWEDISH_COLUMNS: [&str; 5] = ["Datum", "Beskrivning", "Kategori", "Kostnad", "Valuta"];
/// `Description` of the trailing totals row, per language.
pub const TOTAL_DESCRIPTIONS: [&str; 2] = ["Total balance", "Totalsumma"];
/// Member columns per file: the household has exactly two users.
pub const MEMBER_COLUMNS: usize = 2;

/// The export's UI language.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Language {
    English,
    Swedish,
}

impl Language {
    pub fn as_str(self) -> &'static str {
        match self {
            Self::English => "English",
            Self::Swedish => "Swedish",
        }
    }
}

/// The parsed header.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Header {
    pub language: Language,
    /// Member names in header order.
    pub members: Vec<String>,
}

/// One data row (an expense or a payment), its fields as in the file (after CSV unquoting).
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct DataRow {
    /// 1-based line number in the file.
    pub line: u64,
    pub date: String,
    pub description: String,
    pub category: String,
    pub cost: String,
    pub currency: String,
    /// Member nets in header order.
    pub nets: Vec<String>,
    /// 0-based index of this row among identical rows earlier in the file (SPEC §10.8 `<n>`).
    pub occurrence: usize,
}

/// A totals row: the member nets of the whole group, per currency.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct TotalRow {
    pub line: u64,
    pub currency: String,
    /// Member nets in header order.
    pub nets: Vec<String>,
}

/// A row that cannot be read.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct RowProblem {
    pub line: u64,
    pub reason: String,
}

/// A whole parsed file.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ParsedFile {
    pub header: Header,
    pub rows: Vec<DataRow>,
    pub totals: Vec<TotalRow>,
    pub blank_rows: usize,
    pub problems: Vec<RowProblem>,
}

/// Why a file cannot be read at all.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum FileError {
    /// Wrong or missing header (SPEC §10.8: fails the import, writing nothing).
    Header(String),
    /// Not CSV at all (bad UTF-8, broken quoting).
    Csv(String),
}

impl std::fmt::Display for FileError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Header(m) | Self::Csv(m) => f.write_str(m),
        }
    }
}

fn expected_headers() -> String {
    format!(
        "expected \"{}\" (English) or \"{}\" (Swedish), then exactly {MEMBER_COLUMNS} member columns named by full \
         Splitwise name",
        ENGLISH_COLUMNS.join(","),
        SWEDISH_COLUMNS.join(",")
    )
}

/// Checks the header line.
pub fn parse_header(fields: &[String]) -> Result<Header, FileError> {
    let fixed: Vec<&str> = fields.iter().take(5).map(String::as_str).collect();
    let language = if fixed == ENGLISH_COLUMNS {
        Language::English
    } else if fixed == SWEDISH_COLUMNS {
        Language::Swedish
    } else {
        return Err(FileError::Header(format!(
            "unrecognised header; {}",
            expected_headers()
        )));
    };
    let members: Vec<String> = fields.iter().skip(5).map(|m| m.trim().to_owned()).collect();
    if members.len() != MEMBER_COLUMNS {
        return Err(FileError::Header(format!(
            "the header has {} member columns; {}",
            members.len(),
            expected_headers()
        )));
    }
    if members.iter().any(String::is_empty) {
        return Err(FileError::Header("a member column has no name".into()));
    }
    if members[0] == members[1] {
        return Err(FileError::Header(
            "the two member columns have the same name".into(),
        ));
    }
    Ok(Header { language, members })
}

/// The 1-based line number of the record csv reports at byte `offset`. csv reports the offset where it started
/// reading, before the empty lines it skips, so those are stepped over first.
fn line_at(bytes: &[u8], offset: u64) -> u64 {
    let mut end = usize::try_from(offset)
        .unwrap_or(usize::MAX)
        .min(bytes.len());
    while end < bytes.len() && (bytes[end] == b'\n' || bytes[end] == b'\r') {
        end += 1;
    }
    let newlines = bytes[..end].iter().filter(|b| **b == b'\n').count();
    u64::try_from(newlines)
        .unwrap_or(u64::MAX)
        .saturating_add(1)
}

/// Parses a whole export. Rows that do not have the header's column count are problems, not errors, so every bad
/// row is reported at once.
pub fn parse(bytes: &[u8]) -> Result<ParsedFile, FileError> {
    let bytes = bytes.strip_prefix(b"\xEF\xBB\xBF").unwrap_or(bytes);
    let mut reader = csv::ReaderBuilder::new()
        .has_headers(false)
        .flexible(true)
        .from_reader(bytes);
    let mut records = reader.records();
    let header_fields: Vec<String> = match records.next() {
        Some(Ok(record)) => record.iter().map(str::to_owned).collect(),
        Some(Err(e)) => return Err(FileError::Csv(format!("cannot read the header: {e}"))),
        None => {
            return Err(FileError::Header(format!(
                "the file is empty; {}",
                expected_headers()
            )));
        }
    };
    let header = parse_header(&header_fields)?;
    let width = 5 + header.members.len();
    let mut rows = Vec::new();
    let mut totals = Vec::new();
    let mut problems = Vec::new();
    let mut blank_rows = 0;
    let mut occurrences: BTreeMap<Vec<String>, usize> = BTreeMap::new();
    for record in records {
        let record = record.map_err(|e| FileError::Csv(format!("CSV error: {e}")))?;
        // csv's own line counter does not count the empty lines it skips.
        let line = record.position().map_or(0, |p| line_at(bytes, p.byte()));
        let fields: Vec<String> = record.iter().map(str::to_owned).collect();
        if fields.iter().all(|f| f.trim().is_empty()) {
            blank_rows += 1;
            continue;
        }
        if fields.len() != width {
            problems.push(RowProblem {
                line,
                reason: format!("{} fields, the header has {width}", fields.len()),
            });
            continue;
        }
        let nets = fields[5..].to_vec();
        if TOTAL_DESCRIPTIONS.contains(&fields[1].trim()) && fields[3].trim().is_empty() {
            totals.push(TotalRow {
                line,
                currency: fields[4].trim().to_owned(),
                nets,
            });
            continue;
        }
        let count = occurrences.entry(fields.clone()).or_insert(0);
        let occurrence = *count;
        *count += 1;
        rows.push(DataRow {
            line,
            date: fields[0].clone(),
            description: fields[1].clone(),
            category: fields[2].clone(),
            cost: fields[3].clone(),
            currency: fields[4].clone(),
            nets,
            occurrence,
        });
    }
    Ok(ParsedFile {
        header,
        rows,
        totals,
        blank_rows,
        problems,
    })
}

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

    fn fields(line: &str) -> Vec<String> {
        line.split(',').map(str::to_owned).collect()
    }

    #[test]
    fn headers() {
        let en = parse_header(&fields("Date,Description,Category,Cost,Currency,A B,C D")).unwrap();
        assert_eq!(en.language, Language::English);
        assert_eq!(en.members, ["A B", "C D"]);
        let sv =
            parse_header(&fields("Datum,Beskrivning,Kategori,Kostnad,Valuta,C D,A B")).unwrap();
        assert_eq!(sv.language, Language::Swedish);
        assert_eq!(sv.members, ["C D", "A B"]);
        for bad in [
            "Date,Description,Category,Cost",
            "Date,Description,Category,Cost,Currency,A B",
            "Date,Description,Category,Cost,Currency,A B,C D,E F",
            "Datum,Description,Category,Cost,Currency,A B,C D",
            "Date,Description,Category,Cost,Valuta,A B,C D",
            "Date,Description,Category,Cost,Currency,A B,A B",
            "Date,Description,Category,Cost,Currency,A B,",
        ] {
            assert!(
                matches!(parse_header(&fields(bad)), Err(FileError::Header(_))),
                "{bad}"
            );
        }
    }

    #[test]
    fn rows_and_occurrences() {
        let text = "\u{feff}Date,Description,Category,Cost,Currency,A B,C D\n\n\
                    2025-01-01,\"Milk, bread\",Groceries,10.00,SEK,5.00,-5.00\n\
                    2025-01-01,\"Milk, bread\",Groceries,10.00,SEK,5.00,-5.00\n\
                    ,,,,,,\n\
                    2025-01-02,Short,Groceries,10.00,SEK\n\
                    \n\
                    2025-01-03,Total balance, , ,SEK,5.00,-5.00\n";
        let parsed = parse(text.as_bytes()).unwrap();
        assert_eq!(parsed.rows.len(), 2);
        assert_eq!(parsed.rows[0].description, "Milk, bread");
        assert_eq!(parsed.rows[0].occurrence, 0);
        assert_eq!(parsed.rows[1].occurrence, 1);
        assert_eq!(parsed.rows[0].line, 3);
        assert_eq!(parsed.totals.len(), 1);
        assert_eq!(parsed.totals[0].nets, ["5.00", "-5.00"]);
        assert_eq!(parsed.problems.len(), 1);
        assert_eq!(parsed.problems[0].line, 6);
        assert!(parsed.blank_rows >= 1);
    }

    #[test]
    fn empty_and_broken_files() {
        assert!(matches!(parse(b""), Err(FileError::Header(_))));
        assert!(matches!(parse(b"a,b\n"), Err(FileError::Header(_))));
    }
}