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use std::env; // (1) Module import: Compiler resolves symbolic references, linking 
                // against the `env` module's compiled object code (static/dynamic).
                // Triggers inclusion of associated data structures and functions into 
                // the current compilation unit, affecting final binary size.

use std::ffi::OsString;// (2) Deep path resolution: Compiler navigates module namespaces,  
                    // loading necessary type definitions. Ensures type safety at
                    // compile time, optimizing memory layout according to  `OsString`'s
                    // internal representation (likely platform-dependent).

use std::fs::File;    // (3) `File` potentially wraps OS file handles (integers) abstracting
                       //   low-level I/O operations. Syscalls like `open`, `read`, incurred on usage.
                        // Implies runtime overhead for kernel transitions compared to basic stack operations.

use std::io::{self, Read, Write};// (4) Traits define abstract interfaces, promoting code
                         // reusability without compile-time type knowledge. Enables
                            // polymorphism via virtual function tables if dynamic dispatch is needed, increasing 
                             //indirect memory access costs.
const BUF_SIZE: usize = 512; // (5) Compiler replaces `BUF_SIZE` with the literal `512`
                             //  during lexical analysis phase.  Impacts memory
                              // allocation of the `buffer` at a deeper scope (on stack likely).

fn main() {                 // (6) `main` is the entry point (defined by C++ runtime spec.).
                              //   Compiler emits code for stack frame setup(pushing EBP/RBP, ESP/RSP), 
                             // ensuring program state before function entry (essential for proper unwind/return).

    let (dash_s, files) = parse_args();// (7) Tuple construction & destructuring involves move semantics
                                  // potentially avoiding costly copies if  `parse_args` returns temporaries (rvalues).
                                    //   Registers potentially used for passing smaller data (dash_s); stack frame expanded 
                                     //     if files exceeds register capacity.

    
    if files.is_empty() {        // (8) Branching instruction emitted: Conditional check of boolean `files.is_empty()` 
                              //      influencing program counter value to either skip the block or enter (conditional jump).
      cat_stream(&mut io::stdin(), &mut io::stdout(), dash_s);// (9) Function call: Parameters are pushed to stack. x86-64 uses registers initially; then overflows onto the stack).
                              //Calling conventions influence argument layout (e.g., System V AMD64 ABI). Runtime resolves `stdin/stdout` indirection.

        return;                // (10) Stack unwinding occurs: Frame pointer restored (ESP/RSP popped). Potential
                              // flushing/invalidation of cache lines occurs, influenced by hardware specs. Return value likely stored in EAX/RAX register.

    }

    for path in files {     // (11) Iterator abstraction: Underneath, `files` yields iterators implemented as a trait 
                               // object (pointer to virtual function table). Every `path` access indirects
                              //through this table adding minor overhead (compared to direct struct access)


        match File::open(&path) { //(12) Error handling via `Result` type involves discriminant based control flow
                         // with enum layouts managed at compile time (`Ok`, `Err`  mapped to data segments internally).
            Ok(mut file) => {   // (13) Pattern matching;Compiler evaluates discriminant at runtime to select branch.
                           //  Mutable borrow (file): Unique access guaranteed; Compiler enforces this at compilation via borrow checker preventing data races.
                if let Err(e) = cat_stream(&mut file, &mut io::stdout(), dash_s) {  // (14) Nested Error Handling
                       //Runtime evaluates return value from cat_stream to detect possible failure paths (`io::Result` unpacking).
                    if !dash_s {    // (15) Boolean NOT involves inverting a bit (likely with XOR instruction at machine level); 
                          // branch based on  Zero flag or setting based on dash_s truthiness.

                        eprintln!("Error processing file: {}: {}", path.to_string_lossy(), e); // (16) String formatting incurs dynamic memory operations on heap for new strings constructed;
                              // System call involved (writes to stderr - likely file descriptor 2) inducing kernel context switch costs.


                    }
                }
            }
            Err(e) => {     // (17) Compiler ensures exhaustive pattern match of all `Result` variants to ensure safety & well-defined paths at runtime.
                if !dash_s {     // (18) Redundant code may be eliminated if optimizer detects its semantic equivalence
                               //      with the `if !dash_s` condition at block (14), possibly through Common Subexpression Elimination.

                    eprintln!("Error opening file: {}: {}", path.to_string_lossy(), e);// (19) Duplication - potential optimization by function extraction; Compiler might still
                              //   inline if optimization levels are high, incurring inlining overhead with a reduced code footprint trade-off.

                }
            }
        }
    }
}

fn parse_args() -> (bool, Vec<OsString>) {  // (20) Return type implications -  ABI dictates how `bool` (potentially byte/word aligned), 
                                             //    and heap-allocated `Vec`  (data pointer with size/capacity fields) is returned. This impacts how data structures are
                                         //           handled, either in dedicated registers or using address/value transmission on the stack frame.


    let mut dash_s = false;// (21) `dash_s` likely resides on the stack. Might be promoted
                         //   to register if highly used by loop or function to reduce memory accesses at runtime, improving perf.

    let mut files = vec![];// (22) `vec![]` invokes the `Vec::new()` constructor underneath, which will perform 
                          //  an initial heap allocation managed by Rust's memory manager. Minimum size set by allocation strategy.


    
    for arg in env::args_os().skip(1) {// (23) Iterator chain `.skip(1)` induces internal creation of a new adapter iterator without consuming 
                                      //  underlying `env::args_os`. Overhead negligible compared to OS API interaction; However stack frame usage per adapter is relevant.


        if arg == "-s" {          // (24)  String comparison `arg == "-s"` may be optimized to direct
                           //      pointer comparison if both operands are string literals allowing CPU's branch prediction to perform efficiently

            dash_s = true;          // (25) Direct boolean assignment sets internal bit value to 1 within `dash_s`. Change could lead
                         //  to cache line invalidations with CPU architecture effects if this location also shared across other threads

        } else if arg.to_str().map_or(false, |s| s.starts_with('-')) {  // (26) `map_or`: Branching logic generated by the compiler around the 
                        //  Optional type for `to_str`. Short circuiting of `starts_with('-')` evaluation minimizes redundant work improving hot-path perf.
            continue;                // (27) `continue` modifies the loop counter causing an unconditional jump to next iteration. Jump impacts CPU pipeline slightly (predictability dependent).
                                        // CPU prefetcher would make optimizations based on assumed next path

        } else {
            files.push(arg);      // (28) `push` may require heap reallocation. If the current allocation can hold `arg`, realloc avoided (perf boost). 
                            // Capacity preallocation strategies in `Vec` internals matter in cost amortization

        }
    }
    
    (dash_s, files)        // (29) Tuple returned – underlying representation possibly flattened by compiler. 
                            // Potential copy of elements (especially for files' pointer) onto a contiguous
                             //  block in a special stack/register-based temporary slot determined by ABI/compiler.  

}


fn cat_stream<R: Read, W: Write>(// (30) Generics:  Monomorphization by compiler for used `R` and `W` types
                               //at each invocation. Code bloat potential for multiple `R` and `W` combinations (vs virtual functions: runtime overhead)
    input: &mut R,         // (31)  `&mut R` :Mutable references  at a machine code level become pointers to data structures at potentially randomized
                              //memory locations post-ASLR. Data structures (like vtables for virtual calls on trait objects like `Read`) located elsewhere.
    output: &mut W,
suppress_errors: bool, // (32) Value is typically moved/copied to stack location, possibly even to CPU
                        //register for quick/easy lookup with low overhead on every loop iteration (perf. improvement via branch-predict assist)  .

) -> io::Result<()> {         // (33) Return Type convention defines ABI compatibility – error path (Err), data pointer in registers or stack determined at compile time.
                            // Compiler guarantees consistency among all instantiations of this templated function to satisfy the externally observable behavior implied by its declared API
    let mut buffer = [0u8; BUF_SIZE];  // (34) Array `buffer`: `512` contiguous bytes on stack; might span
                         //across multiple cache lines impacting performance if accesses not localized;

    
    loop {                  // (35) Loop implies conditional backward branch (`jmp` instruction at a lower level). Runtime check against the outcome
                        //   of `input.read`, altering control flow (PC manipulation via condition flags evaluation post reading ops).

        let bytes_read = match input.read(&mut buffer) {  // (36) Runtime branch –`match` compiles into conditional
                          //jump instructions (likely involving CMP at assembly). Code organization optimized based on enum tag (discriminant) of `Result`.

            Ok(0) => break,            // (37) `break` involves modifying PC to skip current loop. Jump instructions set for specific locations;  may hinder instruction-level parallelism
                           //with pipelining if branch prediction poor since target determined only upon branch reach
            Ok(n) => n,             // (38) Data moved directly onto the stack (`bytes_read`); compiler may potentially eliminate `bytes_read` variable altogether if further usage only
                          //occurs in a tail position (via optimization – e.g., using value instead of another load instruction), demonstrating register allocation benefits.


            Err(e) if suppress_errors => return Ok(()), //(39) Conditional error suppression :Compiler emits a test for`suppress_errors`. If false, standard error path taken; 
                        //otherwise short-circuits to early return skipping potentially extensive unwind operations of a deep error path

            Err(e) => return Err(e),    // (40) Error propagation. Deeply returning error information, involving 
                              //either copying it to a result struct/register (potential overhead depending on error value size, if it isn't just an integer enum discriminant) or wrapping existing error representation

        };

        let mut bytes_written = 0;      // (41) `bytes_written` initialization to zero translates directly into CPU instructions like `mov` to initialize memory location, consuming CPU cycles
                           //and potentially affecting instruction level parallelism (blocking a write-back until dependencies clear on the execution path).  

        while bytes_written < bytes_read {  //(42) Inner Loop –  Compiled into another conditional branch relying on comparing 
                           //memory location associated to `bytes_written` with `bytes_read` with outcomes impacting instruction pipeline predictability (similar considerations apply to all loops).

            let write_result = output.write(&buffer[bytes_written..bytes_read]);//(43) Slice (`buffer[...]`) calculated at runtime : Implies starting memory address
                                //      for slice start  by offsetting base address via  `bytes_written`, requiring additions potentially at machine code level
                              //    depending on compiler optimizations – impacting arithmetic pipelines  (but potentially minor given common sub-expression/propensity for reordering instructions)


            
            match write_result {            // (44) Duplicate pattern with potential optimization: If compiler detects no external side effects, a single match construct encompassing lines (36) 
                                       //and (44) might be achievable, improving branch prediction (fewer instructions by factoring common branching pathways – a gain from enhanced code locality).  


                Ok(n) => bytes_written += n,// (45) `+=` operator – underlying read-modify-write operation (potential non-atomicity on hardware) incurring memory access and CPU operation latency, affecting execution path.


                Err(e) if suppress_errors => return Ok(()),//(46) Error suppression behavior possibly determined by conditional moves (CMOV) based on `suppress_error`, optimizing branching at CPU level depending on architecture specifics 
                            //(if it avoids a JMP then instruction pipe less sensitive to outcome of condition evaluation).

                Err(e) => return Err(e),  //(47)  Same error semantics as line 40 -- but at deeper level -compiler possibly performs stack-level optimizations 
                       //to collapse the redundant behavior (epilogue/error data copies) depending on knowledge on call paths via static analysis and optimization scope constraints..


            }
        }
    }
    
    Ok(())              // (48) Implies an implicit move operation copying the unit type () to stack/register 
                          //for return handling (as per the specified calling convention ABI constraints). Usually involves zero/no-op moves given () has a trivially determined internal memory layout
                         // with optimization often leading to just register manipulations and/or leaving appropriate value in a predetermined space to be fetched by the calling function).

}