Interview Bank

RAG in 14 Days: From Retrieval to Trustworthy Answers

D3 Getting Documents In: Parsing PDF and HTML, Tables and Scans, Cleaning Rules, and Metadata You Must Keep

• The text extracted from a PDF comes out in the wrong order. How do you diagnose and fix it?一份 PDF 解析出来的文字顺序是乱的，你会怎么排查和修复？

  Common in ChinaCommon overseasIntermediate#pdf-parsing#ingestion#data-quality

  How to reason about it · think before answering

  1. This checks whether you have actually parsed a PDF yourself. The first sentence is the differentiator: a PDF has no reading order at all, only drawing instructions with coordinates.
  2. Start with the diagnostic step: dump the extracted fragments together with page, x, y and font size instead of looking at the concatenated string. The cause is always in the coordinates.
  3. Then classify the symptom. Lines alternating between left and right means multi-column layout was not detected. Fragments with y jumping backwards means the content stream was written in drawing order. Clean text sprinkled with a repeated short line is not disorder at all, it is a header or footer that was never stripped.
  4. Match the fix to the symptom. For columns, rebuild the order: sort the left edges of the fragments on each page, take the widest gap as the column boundary, then sort by column, then y descending, then x ascending. For headers and footers, cut fixed bands at the top and bottom and print how many fragments you dropped so you can confirm you did not cut into the body.
  5. Add the production-grade part: the fix needs a regression signal, not an eyeball check. Compute an out-of-order score by walking the sorted fragments and counting backward jumps within a column plus right-to-left column jumps. It needs no ground truth, so it can run on every ingest.
  6. Expected follow-up: what if column detection is wrong? Keep the detector conservative, treating a narrow gap or a lopsided split as single column, and make sure the assertion still fires when a two-column page is misread as one. Missing a fix is better than silently corrupting the order.

  分析过程 · 先想清楚再作答

  1. 这题在考你有没有真的动手解析过 PDF。区分度在第一句：能不能说出「PDF 里根本没有阅读顺序」这个前提。答不出这句的人，后面只会说「换个库试试」。
  2. 先给排查顺序：把抽出来的文本片段连同页码、坐标、字号一起打印出来，别只看拼好的字符串。乱序的原因几乎都藏在坐标里，看纯文本永远看不出来。
  3. 然后按现象分三类。左右两栏一行一行地交替，是多栏没识别；同一段话被拆成很多短片段且 y 值有回跳，是内容流按绘制顺序写的；文字整体没问题但夹着重复出现的短句，那不是乱序，是页眉页脚没剔。
  4. 修法对应着来：多栏就重建阅读顺序——把每页文字块的左边界排序找最大空隙当分栏线，再按「栏号、y 从大到小、x 从小到大」重排；页眉页脚按固定的 y 值带切掉，并打印剔除条数确认没误伤。
  5. 补一条能证明你在生产里干过的话：修完要有可回归的判据，不能靠肉眼。用乱序疑似度——顺着排好的顺序走一遍，统计「同栏内往回跳」和「从右栏跳回左栏」的比例，它不需要标准答案，可以挂进流水线天天跑。
  6. 可预期的追问：多栏识别错了怎么办？回答分两头——把分栏判定做保守（空隙不够宽、或者一侧内容占比太低就按单栏处理），并且让断言在双栏被误判成单栏时同样会报警，宁可漏修也不要悄悄改错。

  Key points

  • State the premise: a PDF stores only drawing instructions, so paragraphs and reading order are inferred, not read.
  • Debug by dumping fragments with page, coordinates and font size; plain text hides the cause.
  • Three common causes: undetected multi-column layout, content stream written in drawing order, and headers or footers left in.
  • Fix columns by finding the widest gap between left edges and sorting by column, then y descending, then x ascending.
  • Add a ground-truth-free regression metric such as an out-of-order score so the fix stays fixed.

  答题要点

  • 前提先说清：PDF 只存「在某页某坐标画某段文字」，段落和阅读顺序都是解析时推出来的。
  • 排查时把片段连同页码、坐标、字号一起打印，纯文本看不出乱序的原因。
  • 三种典型成因：多栏没识别、内容流按绘制顺序写、页眉页脚没剔除。
  • 多栏的修法是找最大 x 空隙定分栏线，再按「栏号、y 降序、x 升序」重排。
  • 修完要有不依赖标准答案的回归指标，比如乱序疑似度，能挂进摄取流水线。