use crate::ast::TemplateAstExpr;
use crate::input::NomoInput;

pub struct EmitMachine {
    current_index: usize,
}

impl EmitMachine {
    fn reserve_slot(&mut self) -> VariableSlot {
        VariableSlot {
            index: {
                let val = self.current_index;
                self.current_index += 1;
                val
            },
        }
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct VariableSlot {
    index: usize,
}

#[derive(Debug)]
pub enum Instruction {
    AppendContent {
        content: NomoInput,
    },
    LoadFromContextToSlot {
        name: NomoInput,
        slot: VariableSlot,
    },
    EmitFromSlot {
        slot: VariableSlot,
    },
    PushScope {
        inherit_parent: bool,
    },
    Abort,
    JumpIfNotTrue {
        emit_slot: VariableSlot,
        jump: isize,
    },
    Jump {
        jump: isize,
    },
    NoOp,
}

pub fn emit_machine(input: crate::ast::TemplateAst<'_>) -> Vec<Instruction> {
    let mut eval = vec![];

    let mut machine = EmitMachine { current_index: 0 };

    for ast in input.root() {
        emit_ast_expr(&mut machine, &mut eval, ast);
    }

    eval
}

fn emit_ast_expr(
    machine: &mut EmitMachine,
    eval: &mut Vec<Instruction>,
    ast: &TemplateAstExpr<'_>,
) {
    match ast {
        TemplateAstExpr::StaticContent(template_token) => {
            eval.push(Instruction::AppendContent {
                content: template_token.source().clone(),
            });
        }
        TemplateAstExpr::Interpolation {
            prev_whitespace_content,
            expression,
            post_whitespace_content,
        } => {
            if let Some(ws) = prev_whitespace_content {
                eval.push(Instruction::AppendContent {
                    content: ws.source().clone(),
                });
            }

            let emit_slot = machine.reserve_slot();
            emit_expr_load(machine, eval, emit_slot, expression);
            eval.push(Instruction::EmitFromSlot { slot: emit_slot });

            if let Some(ws) = post_whitespace_content {
                eval.push(Instruction::AppendContent {
                    content: ws.source().clone(),
                });
            }
        }
        TemplateAstExpr::ConditionalChain { chain } => {
            let mut chain = chain.iter();

            let mut end_indices = vec![];

            let mut previous_post_whitespace_content: &Option<crate::parser::TemplateToken> = &None;
            let mut previous_jump: Option<usize> = None;

            loop {
                let next = chain.next().unwrap();
                if let Some(ws) = previous_post_whitespace_content {
                    eval.push(Instruction::AppendContent {
                        content: ws.source().clone(),
                    });
                }

                if let TemplateAstExpr::ConditionalContent { content } = &next {
                    for ast in content {
                        emit_ast_expr(machine, eval, ast);
                    }

                    end_indices.push(eval.len());
                    eval.push(Instruction::Jump { jump: isize::MAX });
                } else if let TemplateAstExpr::Block {
                    prev_whitespace_content,
                    post_whitespace_content,
                    expression,
                } = &next
                {
                    previous_post_whitespace_content = post_whitespace_content;
                    if let Some(ws) = prev_whitespace_content {
                        let idx = end_indices.last().copied();
                        eval.insert(
                            idx.unwrap_or(eval.len()),
                            Instruction::AppendContent {
                                content: ws.source().clone(),
                            },
                        );
                        if let Some(idx) = end_indices.last_mut() {
                            *idx += 1;
                        }
                    }

                    if let TemplateAstExpr::IfConditional { expression } = &**expression {
                        let emit_slot = machine.reserve_slot();
                        emit_expr_load(machine, eval, emit_slot, expression);

                        previous_jump = Some(eval.len());
                        eval.push(Instruction::JumpIfNotTrue {
                            emit_slot,
                            jump: isize::MAX,
                        });
                    } else if let TemplateAstExpr::ElseConditional { expression } = &**expression {
                        if let Some(previous_jump) = previous_jump.take() {
                            let new_jump = eval.len() - previous_jump - 1;
                            let Instruction::JumpIfNotTrue { jump, .. } = &mut eval[previous_jump]
                            else {
                                panic!("Jump slot had something that is not a jump?!");
                            };

                            *jump = new_jump as isize;
                        } else {
                            panic!("Got an else without a previous if?");
                        }

                        if let Some(expression) = expression {
                            let emit_slot = machine.reserve_slot();
                            emit_expr_load(machine, eval, emit_slot, expression);

                            previous_jump = Some(eval.len());
                            eval.push(Instruction::JumpIfNotTrue {
                                emit_slot,
                                jump: isize::MAX,
                            });
                        } else {
                            // We don't have to do anything in the else case
                        }
                    } else if let TemplateAstExpr::EndBlock = &**expression {
                        break;
                    }
                }
            }

            if let Some(previous_jump) = previous_jump.take() {
                let new_jump = eval.len() - previous_jump - 1;
                let Instruction::JumpIfNotTrue { jump, .. } = &mut eval[previous_jump] else {
                    panic!("Jump slot had something that is not a jump?!");
                };

                *jump = new_jump as isize;
            }

            if let Some(ws) = previous_post_whitespace_content {
                eval.push(Instruction::AppendContent {
                    content: ws.source().clone(),
                });
            } else {
                eval.push(Instruction::NoOp);
            }

            for index in end_indices {
                let jump = eval.len() - index - 1;
                eval[index] = Instruction::Jump {
                    jump: jump as isize,
                };
            }
        }

        TemplateAstExpr::Block { .. }
        | TemplateAstExpr::EndBlock
        | TemplateAstExpr::IfConditional { .. }
        | TemplateAstExpr::ConditionalContent { .. }
        | TemplateAstExpr::ElseConditional { .. }
        | TemplateAstExpr::Invalid { .. }
        | TemplateAstExpr::VariableAccess { .. } => eval.push(Instruction::Abort),
    }
}

fn emit_expr_load(
    _machine: &mut EmitMachine,
    eval: &mut Vec<Instruction>,
    emit_slot: VariableSlot,
    expression: &TemplateAstExpr<'_>,
) {
    match expression {
        TemplateAstExpr::VariableAccess(template_token) => {
            eval.push(Instruction::LoadFromContextToSlot {
                name: template_token.source().clone(),
                slot: emit_slot,
            });
        }
        TemplateAstExpr::Invalid { .. } => eval.push(Instruction::Abort),
        TemplateAstExpr::StaticContent { .. } | TemplateAstExpr::Interpolation { .. } => {
            unreachable!("Invalid AST here")
        }
        TemplateAstExpr::ConditionalChain { .. } => todo!(),
        TemplateAstExpr::ElseConditional { .. } => todo!(),
        TemplateAstExpr::EndBlock => todo!(),
        TemplateAstExpr::Block { .. } => todo!(),
        TemplateAstExpr::IfConditional { .. } => todo!(),
        TemplateAstExpr::ConditionalContent { .. } => todo!(),
    }
}

#[cfg(test)]
mod tests {
    use crate::emit::emit_machine;

    #[test]
    fn check_simple_variable_interpolation() {
        let input = "Hello {{= world }}";

        let parsed = crate::parser::parse(input.into()).unwrap();

        let ast = crate::ast::parse(parsed.tokens()).unwrap();

        let emit = emit_machine(ast);

        insta::assert_debug_snapshot!(emit, @r#"
        [
            AppendContent {
                content: "Hello" (0..5),
            },
            AppendContent {
                content: " " (5..6),
            },
            LoadFromContextToSlot {
                name: "world" (10..15),
                slot: VariableSlot {
                    index: 0,
                },
            },
            EmitFromSlot {
                slot: VariableSlot {
                    index: 0,
                },
            },
        ]
        "#);
    }

    #[test]
    fn check_if_else_if() {
        let input = "{{ if foo }} foo {{ else if bar }} bar {{ else }} foobar {{ end }}";

        let parsed = crate::parser::parse(input.into()).unwrap();

        let ast = crate::ast::parse(parsed.tokens()).unwrap();

        let emit = emit_machine(ast);

        insta::assert_debug_snapshot!(emit);
    }
}