skillet 0.6.3

Skillet: micro expression language (arithmetic, logical, functions, arrays, conditionals, excel formulas) made in Rust bin cli and server
Documentation 
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use crate::error::Error;
use crate::runtime::utils::is_blank;
use crate::types::Value;

/// Get the byte offset corresponding to a character index, without collecting into Vec<char>.
#[inline]
fn char_byte_offset(s: &str, char_idx: usize) -> usize {
    s.char_indices()
        .nth(char_idx)
        .map(|(i, _)| i)
        .unwrap_or(s.len())
}

/// Count the number of characters without collecting into Vec<char>.
#[inline]
fn char_count(s: &str) -> usize {
    s.chars().count()
}

pub fn exec_string(name: &str, args: &[Value]) -> Result<Value, Error> {
    match name {
        "SUBSTITUTE" => {
            // SUBSTITUTE(text, substr, replacement)
            match (args.get(0), args.get(1), args.get(2)) {
                (Some(Value::String(text)), Some(Value::String(substr)), Some(Value::String(repl))) => {
                    Ok(Value::String(text.replace(substr, repl)))
                }
                _ => Err(Error::new(
                    "SUBSTITUTE expects (text: string, substr: string, replacement: string)",
                    None,
                )),
            }
        }
        "SUBSTITUTEM" => {
            // SUBSTITUTEM(text, substr, replacement) - replace all occurrences (alias of SUBSTITUTE)
            match (args.get(0), args.get(1), args.get(2)) {
                (Some(Value::String(text)), Some(Value::String(substr)), Some(Value::String(repl))) => {
                    Ok(Value::String(text.replace(substr, repl)))
                }
                _ => Err(Error::new(
                    "SUBSTITUTEM expects (text: string, substr: string, replacement: string)",
                    None,
                )),
            }
        }
        "LEFT" => {
            // LEFT(String, [NumberOfCharacters]) -> default 1 character if omitted
            if args.is_empty() {
                return Err(Error::new("LEFT expects string, [num_chars]", None));
            }
            let s = match args.get(0) {
                Some(Value::String(st)) => st,
                _ => return Err(Error::new("LEFT expects string as first argument", None)),
            };
            let n = match args.get(1) {
                Some(Value::Number(n)) => *n,
                Some(_) => return Err(Error::new("LEFT expects number as second argument", None)),
                None => 1.0,
            };
            let take = if n.is_finite() && n > 0.0 { n as usize } else { 0usize };
            let byte_end = char_byte_offset(s, take);
            Ok(Value::String(s[..byte_end].to_string()))
        }
        "RIGHT" => {
            // RIGHT(String, [NumberOfCharacters]) -> default 1 character if omitted
            if args.is_empty() {
                return Err(Error::new("RIGHT expects string, [num_chars]", None));
            }
            let s = match args.get(0) {
                Some(Value::String(st)) => st,
                _ => return Err(Error::new("RIGHT expects string as first argument", None)),
            };
            let n = match args.get(1) {
                Some(Value::Number(n)) => *n,
                Some(_) => return Err(Error::new("RIGHT expects number as second argument", None)),
                None => 1.0,
            };
            let take = if n.is_finite() && n > 0.0 { n as usize } else { 0usize };
            let len = char_count(s);
            let start_char = len.saturating_sub(take);
            let byte_start = char_byte_offset(s, start_char);
            Ok(Value::String(s[byte_start..].to_string()))
        }
        "MID" => {
            // MID(String, StartingPosition [, NumberOfCharacters])
            // StartingPosition is 1-based (Excel-like). If NumberOfCharacters omitted, go to end.
            if args.len() < 2 {
                return Err(Error::new(
                    "MID expects string, start, [num_chars]",
                    None,
                ));
            }
            let s = match args.get(0) {
                Some(Value::String(st)) => st,
                _ => return Err(Error::new("MID expects string as first argument", None)),
            };
            let start_num = match args.get(1) {
                Some(Value::Number(n)) => *n,
                _ => return Err(Error::new("MID expects number as second argument", None)),
            };
            let len_opt = match args.get(2) {
                Some(Value::Number(n)) => Some(*n),
                Some(_) => return Err(Error::new("MID expects number as third argument", None)),
                None => None,
            };

            let total = char_count(s);
            // Excel-like: 1-based start; clamp below 1 to 1
            let start_index = if start_num.is_finite() {
                let s1 = if start_num < 1.0 { 1.0 } else { start_num.floor() } as usize;
                s1.saturating_sub(1).min(total)
            } else {
                0usize
            };
            let end_index = if let Some(n) = len_opt {
                let take = if n.is_finite() && n > 0.0 { n as usize } else { 0usize };
                start_index.saturating_add(take).min(total)
            } else {
                total
            };
            if start_index >= total || start_index >= end_index {
                Ok(Value::String(String::new()))
            } else {
                let byte_start = char_byte_offset(s, start_index);
                let byte_end = char_byte_offset(s, end_index);
                Ok(Value::String(s[byte_start..byte_end].to_string()))
            }
        }
        "LENGTH" => match args.get(0) {
            Some(Value::Array(items)) => Ok(Value::Number(items.len() as f64)),
            Some(Value::String(s)) => Ok(Value::Number(s.chars().count() as f64)),
            Some(Value::Null) => Ok(Value::Number(0.0)),
            Some(_) | None => Err(Error::new("LENGTH expects array or string", None)),
        },
        "CONCAT" => {
            let mut out = String::new();
            fn push_val(s: &mut String, v: &Value) -> Result<(), Error> {
                match v {
                    Value::String(st) => {
                        s.push_str(st);
                        Ok(())
                    }
                    Value::Number(n) => {
                        s.push_str(&n.to_string());
                        Ok(())
                    }
                    Value::Array(arr) => {
                        for it in arr {
                            push_val(s, it)?;
                        }
                        Ok(())
                    }
                    Value::Boolean(b) => {
                        s.push_str(if *b { "TRUE" } else { "FALSE" });
                        Ok(())
                    }
                    Value::Null => Ok(()),
                    Value::Currency(_) => Ok(()),
                    Value::DateTime(_) => Ok(()),
                    Value::Json(_) => Ok(()),
                }
            }
            for a in args {
                if let Value::Null = a { /* skip */
                } else {
                    push_val(&mut out, a)?;
                }
            }
            Ok(Value::String(out))
        }
        "UPPER" => match args.get(0) {
            Some(Value::String(s)) => Ok(Value::String(s.to_uppercase())),
            _ => Err(Error::new("UPPER expects string", None)),
        },
        "LOWER" => match args.get(0) {
            Some(Value::String(s)) => Ok(Value::String(s.to_lowercase())),
            _ => Err(Error::new("LOWER expects string", None)),
        },
        "TRIM" => match args.get(0) {
            Some(Value::String(s)) => Ok(Value::String(s.trim().to_string())),
            _ => Err(Error::new("TRIM expects string", None)),
        },
        "SUBSTRING" => {
            if args.len() < 2 {
                return Err(Error::new(
                    "SUBSTRING expects string, start, [length]",
                    None,
                ));
            }
            let string = match args.get(0) {
                Some(Value::String(s)) => s,
                _ => {
                    return Err(Error::new(
                        "SUBSTRING expects string as first argument",
                        None,
                    ))
                }
            };
            let start = match args.get(1) {
                Some(Value::Number(n)) => *n as usize,
                _ => {
                    return Err(Error::new(
                        "SUBSTRING expects number as second argument",
                        None,
                    ))
                }
            };

            let string_len = char_count(string);

            let end = if let Some(Value::Number(len)) = args.get(2) {
                let length = *len as usize;
                start.saturating_add(length).min(string_len)
            } else {
                string_len
            };

            let start = start.min(string_len);
            let end = end.max(start);

            if start >= string_len {
                Ok(Value::String(String::new()))
            } else {
                let byte_start = char_byte_offset(string, start);
                let byte_end = char_byte_offset(string, end);
                Ok(Value::String(string[byte_start..byte_end].to_string()))
            }
        }
        "SPLIT" => match (args.get(0), args.get(1)) {
            (Some(Value::String(s)), Some(Value::String(sep))) => Ok(Value::Array(
                s.split(sep).map(|p| Value::String(p.to_string())).collect(),
            )),
            (Some(Value::String(s)), None) => Ok(Value::Array(
                s.split(',')
                    .map(|p| Value::String(p.trim().to_string()))
                    .collect(),
            )),
            _ => Err(Error::new("SPLIT expects string, [separator]", None)),
        },
        "REPLACE" => {
            // Excel-like: REPLACE(old_text, start_num, num_chars, new_text)
            // start_num is 1-based; num_chars may be 0; count by Unicode scalar values
            if args.len() != 4 {
                return Err(Error::new(
                    "REPLACE expects (old_text: string, start_num: number, num_chars: number, new_text: string)",
                    None,
                ));
            }
            let old_text = match args.get(0) { Some(Value::String(s)) => s, _ => return Err(Error::new("REPLACE expects string as first argument", None)) };
            let start_num = match args.get(1) { Some(Value::Number(n)) => *n, _ => return Err(Error::new("REPLACE expects number as second argument", None)) };
            let num_chars = match args.get(2) { Some(Value::Number(n)) => *n, _ => return Err(Error::new("REPLACE expects number as third argument", None)) };
            let new_text = match args.get(3) { Some(Value::String(s)) => s, _ => return Err(Error::new("REPLACE expects string as fourth argument", None)) };

            let len = char_count(old_text);

            // Clamp start (1-based) to [1, len+1]
            let start_idx_1b = if start_num.is_finite() { start_num.floor().max(1.0) as usize } else { 1usize };
            let start_idx = start_idx_1b.saturating_sub(1).min(len);

            let take = if num_chars.is_finite() && num_chars > 0.0 { num_chars.floor() as usize } else { 0usize };
            let end_idx = start_idx.saturating_add(take).min(len);

            let byte_start = char_byte_offset(old_text, start_idx);
            let byte_end = char_byte_offset(old_text, end_idx);

            let mut out = String::with_capacity(old_text.len() + new_text.len());
            out.push_str(&old_text[..byte_start]);
            out.push_str(new_text);
            out.push_str(&old_text[byte_end..]);
            Ok(Value::String(out))
        }
        "REVERSE" => match args.get(0) {
            Some(Value::String(s)) => Ok(Value::String(s.chars().rev().collect())),
            _ => Err(Error::new("REVERSE expects string", None)),
        },
        "ISBLANK" => {
            match args.get(0) {
                Some(v) => Ok(Value::Boolean(is_blank(v))),
                None => Ok(Value::Boolean(true)),
            }
        }
        "ISNUMBER" => {
            Ok(Value::Boolean(matches!(
                args.get(0),
                Some(Value::Number(_) | Value::Currency(_))
            )))
        }
        "ISTEXT" => {
            Ok(Value::Boolean(matches!(args.get(0), Some(Value::String(_)))))
        }
        "INCLUDES" => {
            // INCLUDES(string, substring) -> boolean
            if args.len() != 2 {
                return Err(Error::new("INCLUDES expects string, substring", None));
            }
            match (args.get(0), args.get(1)) {
                (Some(Value::String(s)), Some(Value::String(substring))) => {
                    Ok(Value::Boolean(s.contains(substring)))
                }
                (Some(Value::String(_)), Some(_)) => {
                    Err(Error::new("INCLUDES expects string as second argument", None))
                }
                (Some(_), Some(_)) => {
                    Err(Error::new("INCLUDES expects string as first argument", None))
                }
                _ => Err(Error::new("INCLUDES expects string, substring", None)),
            }
        }
        _ => Err(Error::new(
            format!("Unknown string function: {}", name),
            None,
        )),
    }
}