sheeter-engine 0.1.2

//! Sheeter is a data file conversion library and CLI tool.
//!
//! This crate provides functions to convert between CSV and JSON formats.
//! It is designed to be small, fast, and portable, suitable for use on
//! low-end devices such as Android phones. The core idea is simple: take
//! data in one format, understand its structure, and emit it in another
//! format without unnecessary overhead.
//!
//! # Examples
//!
//! Convert CSV to JSON:
//!
//! ```
//! use sheeter_engine::csv_to_json;
//!
//! let csv = "name,age\nAlice,30\nBob,25";
//! let json = csv_to_json(csv).unwrap();
//! assert_eq!(json, r#"[{"name":"Alice","age":"30"},{"name":"Bob","age":"25"}]"#);
//! ```
//!
//! Convert JSON to CSV:
//!
//! ```
//! use sheeter_engine::json_to_csv;
//!
//! let json = r#"[{"name":"Alice","age":"30"},{"name":"Bob","age":"25"}]"#;
//! let csv = json_to_csv(json).unwrap();
//! assert_eq!(csv, "name,age\nAlice,30\nBob,25\n");
//! ```

use std::io::Cursor;

/// Converts a CSV string into a JSON array of objects.
///
/// CSV (Comma-Separated Values) is a simple tabular format where each line is a
/// row and columns are separated by commas. JSON (JavaScript Object Notation) is
/// a hierarchical data format. This function reads the first row of the CSV as
/// the header (column names) and each subsequent row as a data record. It then
/// builds a JSON array where each element is an object mapping header names to
/// the corresponding cell values as strings. This is useful for feeding CSV data
/// into JSON-based systems or APIs, or for further processing with JSON tools.
///
/// Duplicate header names are rejected because they would silently overwrite
/// data in the resulting JSON objects. All cells are represented as strings in
/// the JSON output; type information (numbers, booleans) is not inferred.
///
/// # Errors
///
/// Returns an error if the CSV is malformed (e.g., inconsistent number of columns),
/// if a header name appears more than once, or if JSON serialization fails (which
/// is unlikely for simple string maps).
///
/// # Examples
///
/// Basic conversion:
///
/// ```
/// use sheeter_engine::csv_to_json;
///
/// let csv = "name,age\nAlice,30\nBob,25";
/// let json = csv_to_json(csv).unwrap();
/// assert_eq!(json, r#"[{"name":"Alice","age":"30"},{"name":"Bob","age":"25"}]"#);
/// ```
///
/// Single column:
///
/// ```
/// use sheeter_engine::csv_to_json;
///
/// let csv = "id\n1\n2\n3";
/// let json = csv_to_json(csv).unwrap();
/// assert_eq!(json, r#"[{"id":"1"},{"id":"2"},{"id":"3"}]"#);
/// ```
///
/// Duplicate headers are rejected:
///
/// ```
/// use sheeter_engine::csv_to_json;
///
/// assert!(csv_to_json("a,a\n1,2").is_err());
/// ```
pub fn csv_to_json(csv: &str) -> Result<String, Box<dyn std::error::Error>> {
    let mut rdr = csv::Reader::from_reader(Cursor::new(csv));
    let headers = rdr.headers()?.clone();
    let mut seen = std::collections::HashSet::new();
    for header in headers.iter() {
        if !seen.insert(header) {
            return Err(format!("duplicate CSV header: {header}").into());
        }
    }
    let mut records = Vec::new();
    for result in rdr.records() {
        let record = result?;
        let mut map = serde_json::Map::new();
        for (header, value) in headers.iter().zip(record.iter()) {
            map.insert(
                header.to_string(),
                serde_json::Value::String(value.to_string()),
            );
        }
        records.push(serde_json::Value::Object(map));
    }
    Ok(serde_json::to_string(&records)?)
}

/// Converts a JSON array of objects into a CSV string.
///
/// JSON arrays of objects are a common way to represent tabular data. This function
/// takes such a JSON string and produces a CSV string. The CSV header row is the
/// union of all keys across all objects, in the order they first appear; if a key
/// is missing in a given object, the corresponding cell is left empty. Values are
/// converted to their string representation: strings are written as-is (without
/// quotes), numbers and booleans use their JSON literal forms, and null becomes
/// an empty cell. Nested objects and arrays cannot be represented losslessly in
/// flat CSV and cause an error, ensuring round-trips never silently corrupt data.
///
/// # Errors
///
/// Returns an error if the JSON is not a valid array of objects, if any value is
/// a nested object or array, or if CSV writing fails (e.g., due to I/O errors on
/// the internal buffer).
///
/// # Examples
///
/// Basic conversion:
///
/// ```
/// use sheeter_engine::json_to_csv;
///
/// let json = r#"[{"name":"Alice","age":"30"},{"name":"Bob","age":"25"}]"#;
/// let csv = json_to_csv(json).unwrap();
/// assert_eq!(csv, "name,age\nAlice,30\nBob,25\n");
/// ```
///
/// With numbers and booleans:
///
/// ```
/// use sheeter_engine::json_to_csv;
///
/// let json = r#"[{"id":1,"active":true},{"id":2,"active":false}]"#;
/// let csv = json_to_csv(json).unwrap();
/// assert_eq!(csv, "id,active\n1,true\n2,false\n");
/// ```
///
/// Extra keys in later objects are preserved:
///
/// ```
/// use sheeter_engine::json_to_csv;
///
/// let json = r#"[{"a":1},{"a":2,"b":3}]"#;
/// assert_eq!(json_to_csv(json).unwrap(), "a,b\n1,\n2,3\n");
/// ```
///
/// Nested values are rejected:
///
/// ```
/// use sheeter_engine::json_to_csv;
///
/// assert!(json_to_csv(r#"[{"a":{"nested":true}}]"#).is_err());
/// ```
pub fn json_to_csv(json: &str) -> Result<String, Box<dyn std::error::Error>> {
    let values: Vec<serde_json::Value> = serde_json::from_str(json)?;
    if values.is_empty() {
        return Ok(String::new());
    }
    let mut headers: Vec<String> = Vec::new();
    for value in &values {
        let obj = value.as_object().ok_or("JSON array must contain objects")?;
        for key in obj.keys() {
            if !headers.contains(key) {
                headers.push(key.clone());
            }
        }
    }
    let mut wtr = csv::Writer::from_writer(vec![]);
    wtr.write_record(&headers)?;
    for value in values {
        let obj = value.as_object().ok_or("JSON array must contain objects")?;
        let mut record: Vec<String> = Vec::with_capacity(headers.len());
        for h in &headers {
            let cell = match obj.get(h) {
                Some(serde_json::Value::String(s)) => s.clone(),
                Some(serde_json::Value::Number(n)) => n.to_string(),
                Some(serde_json::Value::Bool(b)) => b.to_string(),
                Some(serde_json::Value::Null) => String::new(),
                Some(serde_json::Value::Object(_)) | Some(serde_json::Value::Array(_)) => {
                    return Err(
                        format!("nested value at key {h} cannot be represented in CSV").into(),
                    );
                }
                None => String::new(),
            };
            record.push(cell);
        }
        wtr.write_record(&record)?;
    }
    let data = wtr.into_inner()?;
    Ok(String::from_utf8(data)?)
}