diff --git a/src/mode/mod.rs b/src/mode/mod.rs
index f681ecb..d250d67 100644
--- a/src/mode/mod.rs
+++ b/src/mode/mod.rs
@@ -1,5 +1,7 @@
 //! Functions for parsing and managing permissions
 
+use std::{error, fmt::Display, num::ParseIntError};
+
 pub fn get_umask() -> u32 {
     let mask = unsafe { libc::umask(0) };
     let umask = unsafe {libc::umask(mask) };
@@ -7,7 +9,7 @@ pub fn get_umask() -> u32 {
 }
 
 #[derive(Debug, PartialEq)]
-pub enum Bits {
+pub enum Bit {
     Suid = 0o4000,
     Sgid = 0o2000,
     Sticky = 0o1000,
@@ -23,14 +25,37 @@ pub enum Bits {
 }
 
 #[derive(Debug, PartialEq)]
-pub enum Op {
+pub enum ParseError {
+    InvalidBit,
+    InvalidDigit,
+    OutsideRange,
+    ParseIntError(ParseIntError),
+    NoOpSet,
+}
+
+impl Display for ParseError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{self:?}")
+    }
+}
+
+impl error::Error for ParseError {}
+
+impl From<ParseIntError> for ParseError {
+    fn from(value: ParseIntError) -> Self {
+        Self::ParseIntError(value)
+    }
+}
+
+#[derive(Debug, PartialEq)]
+enum Op {
     Add,
     Remove,
     Equals,
 }
 
-#[derive(Debug, PartialEq)]
-pub enum Who {
+#[derive(Clone, Debug, PartialEq)]
+enum Who {
     User,
     Group,
     Other,
@@ -40,7 +65,8 @@ pub enum Who {
 pub struct Parser {
     mode: u32,
     op: Option<Op>,
-    who: Option<Who>,
+    who: Vec<Who>,
+    bits: Vec<char>,
 }
 
 impl Default for Parser {
@@ -51,17 +77,257 @@ impl Default for Parser {
         Self {
             mode,
             op: None,
-            who: None,
+            who: vec![],
+            bits: vec![],
         }
     }
 }
 
 impl Parser {
-    fn new(mode: u32) -> Self {
+    pub fn new(mode: u32) -> Self {
         Self {
             mode,
             op: None,
-            who: None,
+            who: vec![],
+            bits: vec![],
         }
     }
+
+    fn parse_octal(&mut self, value: &str) -> Result<u32, ParseError> {
+        let m = u32::from_str_radix(value, 8)?;
+        if m <= 0o7777 {
+            Ok(m)
+        } else {
+            Err(ParseError::OutsideRange)
+        }
+    }
+
+    fn add_who(&mut self, who: Who) -> Result<(), ParseError> {
+        if self.op.is_some() || self.who.contains(&who) || !self.bits.is_empty() {
+            Err(ParseError::InvalidDigit)
+        } else {
+            self.who.push(who);
+            Ok(())
+        }
+    }
+
+    fn set_op(&mut self, op: Op) -> Result<(), ParseError> {
+        if self.op.is_some() || !self.bits.is_empty() {
+            Err(ParseError::InvalidDigit)
+        } else {
+            self.op = Some(op);
+            Ok(())
+        }
+    }
+
+    fn push_bit(&mut self, bit: char) -> Result<(), ParseError> {
+        match self.op {
+            Some(_) => if self.bits.contains(&bit) {
+                return Err(ParseError::InvalidDigit);
+            } else {
+                self.bits.push(bit);
+            },
+            None => return Err(ParseError::NoOpSet),
+        }
+        Ok(())
+    }
+
+    fn push_suid(&mut self) -> Result<(), ParseError> {
+        for w in &self.who {
+            if w == &Who::All || w == &Who::Other {
+                return Err(ParseError::InvalidBit);
+            }
+        }
+        if self.who.is_empty() {
+            return Err(ParseError::InvalidBit);
+        }
+        self.push_bit('s')
+    }
+
+    fn push_sticky(&mut self) -> Result<(), ParseError> {
+        for w in &self.who {
+            if w == &Who::All || w == &Who::User || w == &Who::Group {
+                return Err(ParseError::InvalidBit);
+            }
+        }
+        if self.who.is_empty() {
+            return Err(ParseError::InvalidBit);
+        }
+        self.push_bit('t')
+    }
+
+    fn add_user_bits(&mut self) {
+        for b in &self.bits {
+            match b {
+                'r' => self.mode |= Bit::URead as u32,
+                'w' => self.mode |= Bit::UWrite as u32,
+                'x' => self.mode |= Bit::UExec as u32,
+                's' => self.mode |= Bit::Suid as u32,
+                _ => unreachable!(),
+            }
+        }
+    }
+
+    fn remove_user_bits(&mut self) {
+        for b in &self.bits {
+            match b {
+                'r' => self.mode &= Bit::URead as u32,
+                'w' => self.mode &= Bit::UWrite as u32,
+                'x' => self.mode &= Bit::UExec as u32,
+                's' => self.mode &= Bit::Suid as u32,
+                _ => unreachable!(),
+            }
+        }
+    }
+
+    fn zero_user_bits(&mut self) {
+        self.mode &= 0o4444;
+    }
+
+    fn set_user_bits(&mut self) {
+        match self.op {
+            Some(Op::Add) => self.add_user_bits(),
+            Some(Op::Remove) => self.remove_user_bits(),
+            Some(Op::Equals) => {
+                self.zero_user_bits();
+                self.add_user_bits();
+            },
+            None => {},
+        }
+    }
+
+    fn add_group_bits(&mut self) {
+        for b in &self.bits {
+            match b {
+                'r' => self.mode |= Bit::GRead as u32,
+                'w' => self.mode |= Bit::GWrite as u32,
+                'x' => self.mode |= Bit::GExec as u32,
+                's' => self.mode |= Bit::Sgid as u32,
+                _ => unreachable!(),
+            }
+        }
+    }
+
+    fn remove_group_bits(&mut self) {
+        for b in &self.bits {
+            match b {
+                'r' => self.mode &= Bit::GRead as u32,
+                'w' => self.mode &= Bit::GWrite as u32,
+                'x' => self.mode &= Bit::GExec as u32,
+                's' => self.mode &= Bit::Sgid as u32,
+                _ => unreachable!(),
+            }
+        }
+    }
+
+    fn zero_group_bits(&mut self) {
+        self.mode &= 0o2222;
+    }
+
+    fn set_group_bits(&mut self) {
+        match self.op {
+            Some(Op::Add) => self.add_group_bits(),
+            Some(Op::Remove) => self.remove_group_bits(),
+            Some(Op::Equals) => {
+                self.zero_group_bits();
+                self.add_group_bits();
+            },
+            None => {},
+        }
+    }
+
+    fn add_other_bits(&mut self) {
+        for b in &self.bits {
+            match b {
+                'r' => self.mode |= Bit::ORead as u32,
+                'w' => self.mode |= Bit::OWrite as u32,
+                'x' => self.mode |= Bit::OExec as u32,
+                't' => self.mode |= Bit::Sticky as u32,
+                _ => unreachable!(),
+            }
+        }
+    }
+
+    fn remove_other_bits(&mut self) {
+        for b in &self.bits {
+            match b {
+                'r' => self.mode &= Bit::ORead as u32,
+                'w' => self.mode &= Bit::OWrite as u32,
+                'x' => self.mode &= Bit::OExec as u32,
+                's' => self.mode &= Bit::Sticky as u32,
+                _ => unreachable!(),
+            }
+        }
+    }
+
+    fn zero_other_bits(&mut self) {
+        self.mode &= 0o1111;
+    }
+
+    fn set_other_bits(&mut self) {
+        match self.op {
+            Some(Op::Add) => self.add_other_bits(),
+            Some(Op::Remove) => self.remove_other_bits(),
+            Some(Op::Equals) => {
+                self.zero_other_bits();
+                self.add_other_bits();
+            },
+            None => {},
+        }
+    }
+
+    fn set_bits(&mut self) {
+        for w in self.who.clone() {
+            match w {
+                Who::User => self.set_user_bits(),
+                Who::Group => self.set_group_bits(),
+                Who::Other => self.set_other_bits(),
+                Who::All => {
+                    self.set_user_bits();
+                    self.set_group_bits();
+                    self.set_other_bits();
+                }
+            }
+        }
+    }
+
+    fn reset(&mut self) {
+        self.who.clear();
+        self.op = None;
+        self.bits.clear();
+    }
+
+    pub fn mode(&self) -> u32 {
+        self.mode
+    }
+
+    pub fn parse(&mut self, value: &str) -> Result<u32, ParseError> {
+        match self.parse_octal(value) {
+            Ok(mode) => {
+                self.mode = mode;
+                return Ok(mode)
+            },
+            Err(e) => if e == ParseError::OutsideRange {
+                return Err(e);
+            }
+        }
+        for c in value.chars() {
+            match c {
+                'u' => self.add_who(Who::User)?,
+                'g' => self.add_who(Who::Group)?,
+                'o' => self.add_who(Who::Other)?,
+                'a' => self.add_who(Who::All)?,
+                '-' => self.set_op(Op::Remove)?,
+                '+' => self.set_op(Op::Add)?,
+                '=' => self.set_op(Op::Equals)?,
+                'r' | 'w' | 'x' => self.push_bit(c)?,
+                's' => self.push_suid()?,
+                't' => self.push_sticky()?,
+                _ => return Err(ParseError::InvalidDigit),
+            }
+        }
+        self.set_bits();
+        self.reset();
+        Ok(self.mode)
+    }
 }