/*!
This crate introduces `Problem` type which can be used on high level APIs (e.g. in command line programs or prototypes) for which error handling boils down to:
* reporting error message (e.g. log with `error!` macro),
* aborting program on error other than a bug (e.g. using `panic!` macro),
* bubbling up errors (e.g. with `?`),
* ignoring errors (e.g. using `Result::ok`).

# Usage
The core type of this crate is `Problem`.
It is used to capture error, backtrace (if enabled) and any additional context information and present it in user friendly way via `Display` implementation.

This library also provides many additional extension traits and some functions that make it easy to construct `Problem` type in different situations 
as well as report or abort programs on error.
It is recommended to import all the types and traits via perlude module: `use problem::prelude::*`.

`Problem` stores error cause information as `Box<dyn Error>` to dealy construction of error message to when it is actually needed.
Additionally `Problem` can also store backtrace `String` (if enabled) and a chain of additional context messages as `Vec<String>`.

In order to support conversion from arbitary types implementing `Error` trait, `Problem` does not implement this trait.

# Creating `Problem`
There are multiple ways to crate `Problem` value.

## Using constructor
Using `Problem::from_error(error)` if `error` implements `Error` trait (via `Into<Box<dyn Error>>`).
Note that `String` and `&str` implement `Into<Box<dyn Error>>` so `Problem` can be constructed directly from strings as well.

```rust
use problem::prelude::*;
use std::io;

Problem::from_error(io::Error::new(io::ErrorKind::InvalidInput, "boom!"));
Problem::from_error(Box::new(io::Error::new(io::ErrorKind::InvalidInput, "boom!")));
Problem::from_error("boom!");
```

Use `Problem::from_error_message(error)` if you don't want to give up ownership of error or only want to keep final message string in memory.

```rust
use problem::prelude::*;
use std::io;

let error = io::Error::new(io::ErrorKind::InvalidInput, "boom!");
let problem = Problem::from_error_message(&error);

drop(error);
drop(problem);
```

## Implicitly
Types implementing `Into<Box<dyn Error>>` trait can be converted to `Problem` via `From` trait. `?` will automatically convert types implementing `Error` to `Problem`.

```rust
use problem::prelude::*;

fn foo() -> Result<String, Problem> {
    let str = String::from_utf8(vec![0, 123, 255])?;
    Ok(str)
}

assert_eq!(foo().unwrap_err().to_string(), "invalid utf-8 sequence of 1 bytes from index 2");
```

If `Error::cause` or `Error::source` is available, the error message from cause chain will be displayed.

```rust
use problem::prelude::*;
use std::fmt;
use std::error::Error;

#[derive(Debug)]
struct ErrorWithCause(std::string::FromUtf8Error);

impl fmt::Display for ErrorWithCause {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "bad things happened")
    }
}

impl Error for ErrorWithCause {
    fn cause(&self) -> Option<&dyn Error> {
        Some(&self.0)
    }
}

fn foo() -> Result<String, Problem> {
    let str = String::from_utf8(vec![0, 123, 255]).map_err(ErrorWithCause)?;
    Ok(str)
}

assert_eq!(foo().unwrap_err().to_string(), "bad things happened; caused by: invalid utf-8 sequence of 1 bytes from index 2");
```

## By explicitly mapping `Result`
`Result<T, E>` can be mapped into `Result<T, Problem>` with `.map_problem()` function.

```rust
use problem::prelude::*;

let res: Result<(), &'static str> = Err("oops");
let problem: Result<(), Problem> = res.map_problem();

assert_eq!(problem.unwrap_err().to_string(), "oops");
```

## By conversion of `Option` into `Result`
`Option<T>` can be converted into `Result<T, Problem>` with `.ok_or_problem(problem)` function where `problem` implements `Into<Problem>`.

```rust
use problem::prelude::*;

let opt: Option<()> = None;
let problem: Result<(), Problem> = opt.ok_or_problem("oops");

assert_eq!(problem.unwrap_err().to_string(), "oops");
```

## From `Result` with `Err` containing `Option`
`.map_problem_or(problem)` method is implemented for `Result<O, Option<E>>` and will map to `Result<O, Problem>` with provided problem for `Err(None)` variant.
This may be usefult when working with FFI.

```rust
use problem::prelude::*;

let unknown: Result<(), Option<&'static str>> = Err(None);
let known: Result<(), Option<&'static str>> = Err(Some("oops"));

assert_eq!(unknown.map_problem_or("unknown error").unwrap_err().to_string(), "unknown error");
assert_eq!(known.map_problem_or("unknown error").unwrap_err().to_string(), "oops");
```

There is also `.map_problem_or_else(problem_function)` variant provided that can be used to defer construction of error to error path.

```rust
use problem::prelude::*;

let unknown: Result<(), Option<&'static str>> = Err(None);
let known: Result<(), Option<&'static str>> = Err(Some("oops"));

assert_eq!(unknown.map_problem_or_else(|| "unknown error").unwrap_err().to_string(), "unknown error");
assert_eq!(known.map_problem_or_else(|| "unknown error").unwrap_err().to_string(), "oops");
```

# Adding context to `Problem`
A context information that provides clues on which good path has been taken that led to error can be added to `Problem` object.

Adding context is a good way to convert other error types to `Problem` as well as providing extra information on where that happened.

## On `Result` error
Method `.problem_while(message)` can be called on any `Result` value that error type can be converted to `Problem` to add context message (via `Dispaly` trait).

```rust
use problem::prelude::*;

let res = String::from_utf8(vec![0, 123, 255]);

assert_eq!(res.problem_while("creating string").unwrap_err().to_string(), "while creating string got error caused by: invalid utf-8 sequence of 1 bytes from index 2");
```

There is also `.problem_while_with(message_function)` variant provided that can be used to defer construction of context message to error path.

```rust
use problem::prelude::*;

let res = String::from_utf8(vec![0, 123, 255]);

assert_eq!(res.problem_while_with(|| "creating string").unwrap_err().to_string(), "while creating string got error caused by: invalid utf-8 sequence of 1 bytes from index 2");
```

## Using scope and `?`
Function `in_context_of(message, closure)` can be used to wrap block of code in a closure that returns `Result`.

This is useful when you want to add context to any error that can happen in the block of code and use `?` operator.
The return type of the closure needs to be `Result<T, Problem>` but `?` operator can convert to `Problem` automatically.

```rust
use problem::prelude::*;

let res = in_context_of("processing string", || {
    let _s = String::from_utf8(vec![0, 123, 255])?;
    // do some processing of _s
    Ok(())
});

assert_eq!(res.unwrap_err().to_string(), "while processing string got error caused by: invalid utf-8 sequence of 1 bytes from index 2");
```

There is also `in_context_of_with(message_function, closure)` variant provided that can be used to defer construction of context message to error path.

```rust
use problem::prelude::*;

let res = in_context_of_with(|| "processing string", || {
    let _s = String::from_utf8(vec![0, 123, 255])?;
    // do some processing of _s
    Ok(())
});

assert_eq!(res.unwrap_err().to_string(), "while processing string got error caused by: invalid utf-8 sequence of 1 bytes from index 2");
```

## Nested context
Context methods can be used multiple times to add layers of context.

```rust
use problem::prelude::*;

fn foo() -> Result<String, Problem> {
    let str = String::from_utf8(vec![0, 123, 255])?;
    Ok(str)
}

let res = in_context_of("doing stuff", || {
    let _s = foo().problem_while("running foo")?;
    // do some processing of _s
    Ok(())
});

assert_eq!(res.unwrap_err().to_string(), "while doing stuff, while running foo got error caused by: invalid utf-8 sequence of 1 bytes from index 2");
```

# Aborting program on `Problem`
`panic!(message, problem)` macro can be used directly to abort program execution but error message printed on the screen will be formatted with `Debug` implementation.

This library provides function `format_panic_to_stderr()` to set up hook that will use `eprintln!("{}", message)` to report panics.
Alternatively if `log` feature is enabled (default), function `format_panic_to_error_log()` will set up hook that will log with `error!("{}", message)` to report panics.

Panic hooks will produce backtrace of panic site if enabled via `RUST_BACKTRACE=1` environment variable along of the `Problem` object backtrace collected 
at object construction site.

```noformat
ERROR: Panicked in libcore/slice/mod.rs:2334:5: index 18492 out of range for slice of length 512
```

## Panicking on `Result` with `Problem`
Similarly to `.expect(message)`, method `.or_failed_to(message)` can be used to abort the program via `panic!()` in case of `Err` variant with `Display` formatted
message and error converted to `Problem` to format the error message with `Display` trait.

```rust,should_panic
use problem::prelude::*;
use problem::format_panic_to_stderr;

format_panic_to_stderr();

// Prints message:
let _s = String::from_utf8(vec![0, 123, 255]).or_failed_to("convert string"); // Failed to convert string due to: invalid utf-8 sequence of 1 bytes from index 2
```

## Panicking on `Option`
Similarly to `.ok_or(error)`, method `.or_failed_to(message)` can be used to abort the program via `panic!()` with `Display` formatted message on `None` variant of `Option` type.

```rust,should_panic
use problem::prelude::*;
use problem::format_panic_to_stderr;

format_panic_to_stderr();
let nothing: Option<&'static str> = None;

let _s = nothing.or_failed_to("get something"); // Failed to get something
```

## Panicking on iterators of `Result`
Method `.or_failed_to(message)` can be used to abort the program via `panic!()` with formatted message on iterators with `Result` item when first `Err` 
is encountered otherwise unwrapping the `Ok` value.
The error type will be converted to `Problem` just before panicing.

```rust,should_panic
use problem::prelude::*;
use problem::format_panic_to_stderr;

format_panic_to_stderr();

let results = vec![Ok(1u32), Ok(2u32), Err("oops")];

let _ok: Vec<u32> = results.into_iter()
    .or_failed_to("collect numbers")
    .collect(); // Failed to collect numbers due to: oops
```

# Logging errors
If `log` feature is enabled (default) function `.ok_or_log_warn()` or `.ok_or_log_error()` can be used on `Result` and iterator of `Result` items to convert
`Result` into `Option` while logging `Err` wariants as warnings or errors.

When used on iterators `.flatten()` addaptor can be used to filter out all `Err` variant items after they were logged and converted to `None`.

```rust
use problem::prelude::*;

# #[cfg(feature = "log")]
# fn test_with_log_feature() {
let results = vec![Ok(1u32), Ok(2), Err("oops"), Ok(3), Err("oh"), Ok(4)];

// Logs warning message: Continuing with error oops
// Logs warning message: Continuing with error oh
let ok: Vec<u32> = results.into_iter()
    .ok_or_log_warn()
    .flatten()
    .collect();

assert_eq!(ok.as_slice(), [1, 2, 3, 4]);
# }
#
# #[cfg(feature = "log")]
# test_with_log_feature();
```

# Backtraces
When compiled with `backtrace` feature (default) formatting of backtraces for `Problem` cause and `panic!` locations can be enabled via 
`RUST_BACKTRACE=1` environment variable.

```noformat
Fatal error: thread 'tests::test_panic_format_stderr_problem' panicked at src/lib.rs:657:35 with: Failed to complete processing task due to: while processing object, while processing input data, while parsing input got error caused by: boom!
--- Cause
   0: backtrace::backtrace::trace_unsynchronized::h936094cb968a67c2
             at /Users/jpastuszek/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.13/src/backtrace/mod.rs:57
   1: problem::Problem::from_error::hfdbc5afef77017de
             at /Users/jpastuszek/Documents/problem/src/lib.rs:435
   2: <problem::Problem as core::convert::From<E>>::from::h3b5fdbec33645197
             at /Users/jpastuszek/Documents/problem/src/lib.rs:500
   3: <T as core::convert::Into<U>>::into::h37311b4bc5720d6d
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libcore/convert.rs:455
   4: <core::result::Result<O, E> as problem::ProblemWhile>::problem_while::{{closure}}::h97ad232ce9ba14fb
             at /Users/jpastuszek/Documents/problem/src/lib.rs:617
   5: <core::result::Result<T, E>>::map_err::he22546342a0a16ff
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libcore/result.rs:530
   6: <core::result::Result<O, E> as problem::ProblemWhile>::problem_while::he5e05f693d81f439
             at /Users/jpastuszek/Documents/problem/src/lib.rs:617
   7: problem::tests::test_panic_format_stderr_problem::he7f271034488edfe
             at /Users/jpastuszek/Documents/problem/src/lib.rs:1053
   8: problem::tests::test_panic_format_stderr_problem::{{closure}}::hd06e112465364a39
             at /Users/jpastuszek/Documents/problem/src/lib.rs:1051
   9: core::ops::function::FnOnce::call_once::hb512c264f284bc3f
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libcore/ops/function.rs:238
  10: <F as alloc::boxed::FnBox<A>>::call_box::h0b961cc85c049bee
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/liballoc/boxed.rs:673
  11: ___rust_maybe_catch_panic
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libpanic_unwind/lib.rs:102
  12: std::sys_common::backtrace::__rust_begin_short_backtrace::h07f538384f587585
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libtest/lib.rs:1426
  13: std::panicking::try::do_call::h4953be8a0738d6ec
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/panicking.rs:310
  14: ___rust_maybe_catch_panic
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libpanic_unwind/lib.rs:102
  15: <F as alloc::boxed::FnBox<A>>::call_box::h5a5d3b35dc8857f3
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/thread/mod.rs:476
  16: std::sys::unix::thread::Thread::new::thread_start::hb66fd5e16b37cfd7
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/sys_common/thread.rs:24
  17: __pthread_body
  18: __pthread_start
--- Panicked
   0: backtrace::backtrace::trace_unsynchronized::h936094cb968a67c2
             at /Users/jpastuszek/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.13/src/backtrace/mod.rs:57
   1: problem::format_panic_to_stderr::{{closure}}::h24ae835f59658f26
             at /Users/jpastuszek/Documents/problem/src/lib.rs:868
   2: std::panicking::rust_panic_with_hook::h3fe6a67edb032589
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/panicking.rs:495
   3: std::panicking::continue_panic_fmt::hf7169aba6b1afe9c
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/panicking.rs:398
   4: std::panicking::begin_panic_fmt::hb5f6d46d54559b8a
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/panicking.rs:353
   5: <core::result::Result<O, E> as problem::FailedTo<O>>::or_failed_to::{{closure}}::h8e0b5d62111b80f4
             at /Users/jpastuszek/Documents/problem/src/lib.rs:657
   6: <core::result::Result<T, E>>::unwrap_or_else::h8bcc063ecb00981e
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libcore/result.rs:774
   7: <core::result::Result<O, E> as problem::FailedTo<O>>::or_failed_to::h2e59dbec5efe6366
             at /Users/jpastuszek/Documents/problem/src/lib.rs:657
   8: problem::tests::test_panic_format_stderr_problem::he7f271034488edfe
             at /Users/jpastuszek/Documents/problem/src/lib.rs:1058
   9: problem::tests::test_panic_format_stderr_problem::{{closure}}::hd06e112465364a39
             at /Users/jpastuszek/Documents/problem/src/lib.rs:1051
  10: core::ops::function::FnOnce::call_once::hb512c264f284bc3f
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libcore/ops/function.rs:238
  11: <F as alloc::boxed::FnBox<A>>::call_box::h0b961cc85c049bee
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/liballoc/boxed.rs:673
  12: ___rust_maybe_catch_panic
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libpanic_unwind/lib.rs:102
  13: std::sys_common::backtrace::__rust_begin_short_backtrace::h07f538384f587585
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libtest/lib.rs:1426
  14: std::panicking::try::do_call::h4953be8a0738d6ec
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/panicking.rs:310
  15: ___rust_maybe_catch_panic
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libpanic_unwind/lib.rs:102
  16: <F as alloc::boxed::FnBox<A>>::call_box::h5a5d3b35dc8857f3
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/thread/mod.rs:476
  17: std::sys::unix::thread::Thread::new::thread_start::hb66fd5e16b37cfd7
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libstd/sys_common/thread.rs:24
  18: __pthread_body
  19: __pthread_start
```

## Access
Formatted backtrace `&str` can be accessed via `Problem::backtrace` function that will return `Some` if `backtrace` feature is enabled and `RUST_BACKTRACE=1` 
environment variable is set.

```rust
use problem::prelude::*;

Problem::from_error("foo").backtrace(); // Some("   0: backtrace...")
```
 */
#[cfg(feature = "log")]
#[macro_use]
extern crate log;
use std::error::Error;
use std::fmt::{self, Display, Write};
use std::panic;

/// Includes `Problem` type and related conversion traits and `in_context_of*` functions
pub mod prelude {
    pub use super::{
        in_context_of, in_context_of_with, FailedTo, FailedToIter, MapProblem, MapProblemOr,
        OkOrProblem, Problem, ProblemWhile,
    };

    #[cfg(feature = "log")]
    pub use super::logged::{OkOrLog, OkOrLogIter};
}

/// Wraps error, context and backtrace information and formats it for display.
/// Data is heap allocated to avoid type parameters or lifetimes.
#[derive(Debug)]
pub struct Problem {
    error: Box<dyn Error>,
    context: Vec<String>,
    backtrace: Option<String>,
}

impl Problem {
    /// Create `Problem` from types implementing `Into<Box<dyn Error>>` (including `String` and `&str`) so that `Error::cause`
    /// chain is followed through in the `Display` message
    pub fn from_error(error: impl Into<Box<dyn Error>>) -> Problem {
        Problem {
            error: error.into(),
            context: Vec::new(),
            backtrace: format_backtrace(),
        }
    }

    /// Same as `Problem::from_error` but stores only final message as `String` and does not take ownership of the error
    pub fn from_error_message(error: &impl Error) -> Problem {
        let mut message = String::new();
        write_error_message(error, &mut message).unwrap();

        Problem {
            error: message.into(),
            context: Vec::new(),
            backtrace: format_backtrace(),
        }
    }

    /// Get backtrace associated with this `Problem` instance if available
    pub fn backtrace(&self) -> Option<&str> {
        self.backtrace.as_ref().map(String::as_str)
    }
}

#[allow(deprecated)]
fn write_error_message(error: &Error, w: &mut impl Write) -> fmt::Result {
    write!(w, "{}", error)?;

    let mut error_cause = error;
    loop {
        // Note: using Error::cause() here to be backward compatible with older errors
        if let Some(cause) = error_cause.cause() {
            write!(w, "; caused by: {}", cause)?;
            error_cause = cause;
        } else {
            break;
        }
    }
    Ok(())
}

impl Display for Problem {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        if let Some(context) = self.context.last() {
            write!(f, "while {}", context)?;
        }
        for context in self.context.iter().rev().skip(1) {
            write!(f, ", while {}", context)?;
        }
        if !self.context.is_empty() {
            write!(f, " got error caused by: ")?;
        }

        write_error_message(self.error.as_ref(), f)?;

        if let Some(backtrace) = self.backtrace.as_ref() {
            write!(f, "\n--- Cause\n{}", backtrace)?;
        }

        Ok(())
    }
}

/// Every type implementing `Into<Box<dyn Error>>` trait (including `String` and `&str` types) can be converted to `Problem` via `?` operator
impl<E> From<E> for Problem
where
    E: Into<Box<dyn Error>>,
{
    fn from(error: E) -> Problem {
        Problem::from_error(error)
    }
}

/// Map type containing error to type containing `Problem`
pub trait MapProblem {
    type ProblemCarrier;
    fn map_problem(self) -> Self::ProblemCarrier;
}

/// Mapping `Result` with error to `Result` with `Problem`
impl<O, E> MapProblem for Result<O, E>
where
    E: Into<Problem>,
{
    type ProblemCarrier = Result<O, Problem>;
    fn map_problem(self) -> Result<O, Problem> {
        self.map_err(|e| e.into())
    }
}

/// Map type not containing any error to type containing given `Problem`
pub trait MapProblemOr {
    type ProblemCarrier;
    fn map_problem_or(self, problem: impl Into<Problem>) -> Self::ProblemCarrier;
    fn map_problem_or_else<F, P>(self, problem: F) -> Self::ProblemCarrier
    where
        F: FnOnce() -> P,
        P: Into<Problem>;
}

/// Mapping `Result` with `Option<E>` to `Result` with `Problem` where `E` implements `Into<Problem>`
impl<O, E> MapProblemOr for Result<O, Option<E>>
where
    E: Into<Problem>,
{
    type ProblemCarrier = Result<O, Problem>;

    fn map_problem_or(self, problem: impl Into<Problem>) -> Result<O, Problem> {
        self.map_err(|e| e.map(Into::into).unwrap_or_else(|| problem.into()))
    }

    fn map_problem_or_else<F, P>(self, problem: F) -> Self::ProblemCarrier
    where
        F: FnOnce() -> P,
        P: Into<Problem>,
    {
        self.map_err(|e| e.map(Into::into).unwrap_or_else(|| problem().into()))
    }
}

/// Extension trait to map `Option` to `Result` with `Problem`
pub trait OkOrProblem<O> {
    fn ok_or_problem<P>(self, problem: P) -> Result<O, Problem>
    where
        P: Into<Problem>;
    fn ok_or_problem_with<F, P>(self, problem: F) -> Result<O, Problem>
    where
        F: FnOnce() -> P,
        P: Into<Problem>;
}

impl<O> OkOrProblem<O> for Option<O> {
    fn ok_or_problem<P>(self, problem: P) -> Result<O, Problem>
    where
        P: Into<Problem>,
    {
        self.ok_or_else(|| problem.into())
    }

    fn ok_or_problem_with<F, P>(self, problem: F) -> Result<O, Problem>
    where
        F: FnOnce() -> P,
        P: Into<Problem>,
    {
        self.ok_or_else(|| problem().into())
    }
}

/// Convert to `Problem` if needed and add context to it
pub trait ProblemWhile {
    type WithContext;

    /// Add context information
    fn problem_while(self, message: impl ToString) -> Self::WithContext;

    /// Add context information from function call
    fn problem_while_with<F, M>(self, message: F) -> Self::WithContext
    where
        F: FnOnce() -> M,
        M: ToString;
}

impl ProblemWhile for Problem {
    type WithContext = Problem;

    fn problem_while(mut self, message: impl ToString) -> Problem {
        self.context.push(message.to_string());
        self
    }

    fn problem_while_with<F, M>(self, message: F) -> Problem
    where
        F: FnOnce() -> M,
        M: ToString,
    {
        self.problem_while(message())
    }
}

impl<O, E> ProblemWhile for Result<O, E>
where
    E: Into<Problem>,
{
    type WithContext = Result<O, Problem>;

    fn problem_while(self, message: impl ToString) -> Result<O, Problem> {
        self.map_err(|err| err.into().problem_while(message))
    }

    fn problem_while_with<F, M>(self, message: F) -> Result<O, Problem>
    where
        F: FnOnce() -> M,
        M: ToString,
    {
        self.map_err(|err| err.into().problem_while_with(message))
    }
}

/// Executes closure with `problem_while` context
pub fn in_context_of<O, B>(message: &str, body: B) -> Result<O, Problem>
where
    B: FnOnce() -> Result<O, Problem>,
{
    body().problem_while(message)
}

/// Executes closure with `problem_while_with` context
pub fn in_context_of_with<O, F, M, B>(message: F, body: B) -> Result<O, Problem>
where
    F: FnOnce() -> M,
    M: ToString,
    B: FnOnce() -> Result<O, Problem>,
{
    body().problem_while_with(message)
}

/// Extension of `Result` that allows program to panic with `Display` message on `Err` for fatal application errors that are not bugs
pub trait FailedTo<O> {
    fn or_failed_to(self, message: impl Display) -> O;
}

impl<O, E> FailedTo<O> for Result<O, E>
where
    E: Into<Problem>,
{
    fn or_failed_to(self, message: impl Display) -> O {
        self.unwrap_or_else(|err| panic!("Failed to {} due to: {}", message, err.into()))
    }
}

impl<O> FailedTo<O> for Option<O> {
    fn or_failed_to(self, message: impl Display) -> O {
        self.unwrap_or_else(|| panic!("Failed to {}", message))
    }
}

/// Iterator that will panic on first error with message displaying `Display` formatted message
pub struct ProblemIter<I, M> {
    inner: I,
    message: M,
}

impl<I, O, E, M> Iterator for ProblemIter<I, M>
where
    I: Iterator<Item = Result<O, E>>,
    E: Into<Problem>,
    M: Display,
{
    type Item = O;

    fn next(&mut self) -> Option<Self::Item> {
        self.inner.next().map(|res| res.or_failed_to(&self.message))
    }
}

/// Convert `Iterator` of `Result<O, E>` to iterator of `O` and panic on first `E` with problem message
pub trait FailedToIter<O, E, M>: Sized {
    fn or_failed_to(self, message: M) -> ProblemIter<Self, M>;
}

impl<I, O, E, M> FailedToIter<O, E, M> for I
where
    I: Iterator<Item = Result<O, E>>,
    E: Into<Problem>,
    M: Display,
{
    fn or_failed_to(self, message: M) -> ProblemIter<Self, M> {
        ProblemIter {
            inner: self,
            message: message,
        }
    }
}

#[cfg(feature = "log")]
pub mod logged {
    use super::*;
    use log::{error, warn};

    /// Extension of `Result` that allows program to log on `Err` with `Display` message for application errors that are not critical
    pub trait OkOrLog<O> {
        fn ok_or_log_warn(self) -> Option<O>;
        fn ok_or_log_error(self) -> Option<O>;
    }

    impl<O, E> OkOrLog<O> for Result<O, E>
    where
        E: Into<Problem>,
    {
        fn ok_or_log_warn(self) -> Option<O> {
            self.map_err(|err| warn!("Continuing with error: {}", err.into()))
                .ok()
        }

        fn ok_or_log_error(self) -> Option<O> {
            self.map_err(|err| error!("Continuing with error: {}", err.into()))
                .ok()
        }
    }

    /// Iterator that will log as warn `Display` formatted message on `Err` and skip to next item; it can be flattened to skip failed items
    pub struct ProblemWarnLoggingIter<I> {
        inner: I,
    }

    impl<I, O, E> Iterator for ProblemWarnLoggingIter<I>
    where
        I: Iterator<Item = Result<O, E>>,
        E: Into<Problem>,
    {
        type Item = Option<O>;

        fn next(&mut self) -> Option<Self::Item> {
            self.inner.next().map(|res| res.ok_or_log_warn())
        }
    }

    /// Iterator that will log as error `Display` formatted message on `Err` and skip to next item; it can be flattened to skip failed items
    pub struct ProblemErrorLoggingIter<I> {
        inner: I,
    }

    impl<I, O, E> Iterator for ProblemErrorLoggingIter<I>
    where
        I: Iterator<Item = Result<O, E>>,
        E: Into<Problem>,
    {
        type Item = Option<O>;

        fn next(&mut self) -> Option<Self::Item> {
            self.inner.next().map(|res| res.ok_or_log_error())
        }
    }

    /// Convert `Iterator` of `Result<O, E>` to iterator of `Option<O>` and log any `Err` variants
    pub trait OkOrLogIter<O, E>: Sized {
        fn ok_or_log_warn(self) -> ProblemWarnLoggingIter<Self>;
        fn ok_or_log_error(self) -> ProblemErrorLoggingIter<Self>;
    }

    impl<I, O, E> OkOrLogIter<O, E> for I
    where
        I: Iterator<Item = Result<O, E>>,
        E: Into<Problem>,
    {
        fn ok_or_log_warn(self) -> ProblemWarnLoggingIter<Self> {
            ProblemWarnLoggingIter { inner: self }
        }

        fn ok_or_log_error(self) -> ProblemErrorLoggingIter<Self> {
            ProblemErrorLoggingIter { inner: self }
        }
    }
}

#[cfg(not(feature = "backtrace"))]
fn format_backtrace() -> Option<String> {
    None
}

/* Use default Rust format like:
   0: std::sys_common::backtrace::_print
             at C:\projects\rust\src\libstd\sys_common\backtrace.rs:94
   1: std::sys_common::backtrace::_print
             at C:\projects\rust\src\libstd\sys_common\backtrace.rs:71
*/
#[cfg(feature = "backtrace")]
#[inline(always)]
fn format_backtrace() -> Option<String> {
    if let Ok("1") = std::env::var("RUST_BACKTRACE").as_ref().map(String::as_str) {
        let mut backtrace = String::new();
        let mut frame_no: u32 = 0;

        backtrace::trace(|frame| {
            let ip = frame.ip();

            if frame_no > 0 {
                backtrace.push_str("\n");
            }

            backtrace::resolve(ip, |symbol| {
                if let Some(name) = symbol.name() {
                    write!(backtrace, "{:4}: {}", frame_no, name).unwrap();
                }
                if let (Some(filename), Some(lineno)) = (symbol.filename(), symbol.lineno()) {
                    write!(
                        backtrace,
                        "\n             at {}:{}",
                        filename.display(),
                        lineno
                    )
                    .unwrap();
                }
            });

            frame_no += 1;
            true // keep going to the next frame
        });

        Some(backtrace)
    } else {
        None
    }
}

fn format_panic(panic: &std::panic::PanicInfo, backtrace: Option<String>) -> String {
    let mut message = String::new();

    let thread = std::thread::current();
    let name = thread.name().unwrap_or("<unnamed>");

    // taken from libstd
    let msg = match panic.payload().downcast_ref::<&'static str>() {
        Some(s) => *s,
        None => match panic.payload().downcast_ref::<String>() {
            Some(s) => &s[..],
            None => "Box<Any>",
        }
    };

    match (backtrace.is_some(), panic.location()) {
        (true, Some(location)) => write!(message, "thread '{}' panicked at {} with: {}", name, location, msg).ok(),
        (true, None) => write!(message, "thread '{}' panicked with: {}", name, msg).ok(),
        (false, _) => write!(message, "{}", msg).ok(),
    };

    if let Some(backtrace) = backtrace {
        message.push_str("\n--- Panicked\n");
        message.push_str(&backtrace);
    }

    message
}

/// Set panic hook so that formats error message to `stderr` with more `Problem` friendly way
pub fn format_panic_to_stderr() {
    panic::set_hook(Box::new(|panic_info| {
        let backtrace = format_backtrace();
        eprintln!("Fatal error: {}", format_panic(panic_info, backtrace));
    }));
}

/// Set panic hook so that when program panics it will log error massage with `error!` macro
#[cfg(feature = "log")]
pub fn format_panic_to_error_log() {
    panic::set_hook(Box::new(|panic_info| {
        let backtrace = format_backtrace();
        error!("{}", format_panic(panic_info, backtrace));
    }));
}

#[cfg(test)]
mod tests {
    use super::prelude::*;
    use super::{format_panic_to_stderr, in_context_of};
    use std::error::Error;
    use std::fmt::{self, Display};
    use std::io;

    #[derive(Debug)]
    struct Foo;

    impl Display for Foo {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Foo error")
        }
    }

    impl Error for Foo {}

    #[derive(Debug)]
    struct Bar(Foo);

    impl Display for Bar {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Bar error")
        }
    }

    impl Error for Bar {
        fn source(&self) -> Option<&(dyn Error + 'static)> {
            Some(&self.0)
        }
    }

    #[derive(Debug)]
    struct Baz(Bar);

    impl Display for Baz {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "Baz error")
        }
    }

    impl Error for Baz {
        fn source(&self) -> Option<&(dyn Error + 'static)> {
            Some(&self.0)
        }
    }

    #[test]
    fn test_convertion() {
        let _: Problem = io::Error::new(io::ErrorKind::InvalidInput, "boom!").into();
        let _: Problem = "boom!".into(); // via impl<'a> From<&'a str> for Box<dyn Error>
    }

    #[test]
    #[should_panic(
        expected = "Failed to complete processing task due to: while processing object, while processing input data, while parsing input got error caused by: boom!"
    )]
    fn test_integration() {
        Err(io::Error::new(io::ErrorKind::InvalidInput, "boom!"))
            .problem_while("parsing input")
            .problem_while("processing input data")
            .problem_while("processing object")
            .or_failed_to("complete processing task")
    }

    #[test]
    #[should_panic(
        expected = "Failed to complete processing task due to: while processing object, while processing input data, while parsing input got error caused by: boom!"
    )]
    fn test_integration_message() {
        Err("boom!")
            .problem_while("parsing input")
            .problem_while("processing input data")
            .problem_while("processing object")
            .or_failed_to("complete processing task")
    }

    #[test]
    #[should_panic(
        expected = "Failed to complete processing task due to: while processing object, while processing input data, while parsing input got error caused by: Baz error; caused by: Bar error; caused by: Foo error"
    )]
    fn test_integration_cause_chain() {
        Err(Baz(Bar(Foo)))
            .problem_while("parsing input")
            .problem_while("processing input data")
            .problem_while("processing object")
            .or_failed_to("complete processing task")
    }

    #[test]
    #[should_panic(
        expected = "Failed to complete processing task due to: while doing stuff got error caused by: boom!"
    )]
    fn test_in_context_of() {
        in_context_of("doing stuff", || {
            Err(io::Error::new(io::ErrorKind::InvalidInput, "boom!"))?
        })
        .or_failed_to("complete processing task")
    }

    #[test]
    #[should_panic(expected = "Failed to foo due to: boom!")]
    fn test_result() {
        Err(io::Error::new(io::ErrorKind::InvalidInput, "boom!")).or_failed_to("foo")
    }

    #[test]
    #[should_panic(
        expected = "Failed to quix due to: Baz error; caused by: Bar error; caused by: Foo error"
    )]
    fn test_result_cause_chain() {
        Err(Baz(Bar(Foo))).or_failed_to("quix")
    }

    #[test]
    #[should_panic(
        expected = "Failed to quix due to: Baz error; caused by: Bar error; caused by: Foo error"
    )]
    fn test_result_cause_chain_message() {
        let error = Baz(Bar(Foo));
        Err(Problem::from_error_message(&error)).or_failed_to("quix")
    }

    #[test]
    #[should_panic(expected = "Failed to foo")]
    fn test_option() {
        None.or_failed_to("foo")
    }

    #[test]
    #[should_panic(expected = "Failed to foo due to: boom!")]
    fn test_option_errors() {
        Err(Some(io::Error::new(io::ErrorKind::InvalidInput, "boom!")))
            .map_problem_or("<unknown error>")
            .or_failed_to("foo")
    }

    #[test]
    #[should_panic(expected = "Failed to foo due to: <unknown error>")]
    fn test_result_option_errors_unknown() {
        let err: Result<(), Option<io::Error>> = Err(None);
        err.map_problem_or("<unknown error>").or_failed_to("foo")
    }

    #[test]
    #[should_panic(expected = "Failed to foo due to: nothing here")]
    fn test_result_ok_or_problem() {
        None.ok_or_problem("nothing here").or_failed_to("foo")
    }

    #[test]
    #[should_panic(expected = "Failed to foo due to: omg!")]
    fn test_result_iter_or_failed_to() {
        let results = vec![Ok(1u32), Ok(2u32), Err("omg!")];
        let _ok = results.into_iter().or_failed_to("foo").collect::<Vec<_>>();
    }

    #[test]
    #[should_panic]
    fn test_panic_format_stderr() {
        format_panic_to_stderr();
        panic!("foo bar!");
    }

    #[test]
    #[should_panic]
    fn test_panic_format_stderr_problem() {
        format_panic_to_stderr();
        let result: Result<(), Problem> = Err(io::Error::new(io::ErrorKind::InvalidInput, "boom!"))
            .problem_while("parsing input")
            .problem_while("processing input data")
            .problem_while("processing object");

        result.or_failed_to("complete processing task");
    }

    #[test]
    #[should_panic]
    fn test_panic_format_stderr_unwrap() {
        format_panic_to_stderr();
        let result: Result<(), io::Error> = Err(io::Error::new(io::ErrorKind::InvalidInput, "boom!"));
        result.unwrap();
    }

    #[test]
    #[should_panic]
    fn test_panic_format_stderr_expect() {
        format_panic_to_stderr();
        let result: Result<(), io::Error> = Err(io::Error::new(io::ErrorKind::InvalidInput, "boom!"));
        result.expect("foo");
    }

    #[test]
    #[cfg(feature = "backtrace")]
    fn test_problem_backtrace() {
        let p = Problem::from_error("foo")
            .problem_while("bar")
            .problem_while("baz");

        if let Ok("1") = std::env::var("RUST_BACKTRACE").as_ref().map(String::as_str) {
            assert!(p.backtrace().is_some());
            println!("{}", p.backtrace().unwrap());
        } else {
            assert!(p.backtrace().is_none());
        }
    }

    #[test]
    #[cfg(feature = "log")]
    fn test_problem_log_error() {
        loggerv::init_quiet().ok();
        let error: Result<(), _> = Err(Baz(Bar(Foo)));
        error.ok_or_log_error();
    }

    #[test]
    #[cfg(feature = "log")]
    fn test_problem_log_warn() {
        loggerv::init_quiet().ok();
        let error: Result<(), _> = Err(Baz(Bar(Foo)));
        error.ok_or_log_warn();
    }

    #[test]
    #[cfg(feature = "log")]
    fn test_problem_log_iter_error() {
        loggerv::init_quiet().ok();
        assert_eq!(
            vec![Ok(1), Err(Foo), Err(Foo), Ok(2), Err(Foo), Ok(3)]
                .into_iter()
                .ok_or_log_error()
                .flatten()
                .collect::<Vec<_>>(),
            vec![1, 2, 3]
        );
    }

    #[test]
    #[cfg(feature = "log")]
    fn test_problem_log_iter_warn() {
        loggerv::init_quiet().ok();
        assert_eq!(
            vec![Ok(1), Err(Foo), Err(Foo), Ok(2), Err(Foo), Ok(3)]
                .into_iter()
                .ok_or_log_warn()
                .flatten()
                .collect::<Vec<_>>(),
            vec![1, 2, 3]
        );
    }
}